I believe the other answers are more in depth; but I wanted to add a bit of nuance to the foundation of your question.
- Chuck lost his career. He loves the law (more than his own brother). This is a big personal loss for Chuck.
- Chuck's hurtful statement to Jimmy was not, in any way, a win for Chuck. It'd be more accurate to call it a necessary evil.
- Chuck's realization that his illness is imaginary also brings with it a burden of guilt, because of everything that Jimmy has done for Chuck during his perceived illness.
Loss of livelihood, loss of his only living family, immense ammounts of self-blame and guilt. This is a recipe for suicide. To his mind, Chuck has nothing to live for.
He manages to extort a huge amount of severance pay ($3 millions) from Howard Hamlin, even Howard says "You won".
Pedantic comment: Chuck wins 8 million. He suggested that his 1/3 share of HHM amounts to around 8 million. When Howard gives Chuck the check for 3 million, he also mentions that it is the first of three checks. it stands to reason that Howard is paying Chuck the 8 million of his share, in three separate payments.
Hamlin's statement was closer to "I give up" than telling Chuck he wins.
Furthermore, Chuck didn't actually win. You can clearly read Chuck's intentions from the list of possible outcomes that he mentions to the board:
- I will sue you and I will ruin HHM's reputation (which will endanger the company's future)
- I will get my share (which will drive HHM into bankrupcy and endanger it's future)
- Let's just forget this all happened and let's be friends again.
Chuck's message is very simple: If I go down, so does HHM. Make your choice.
Chuck presents option 1 and 2 not because he wants it, but because they are deterrents. Chuck doesn't want to get pushed out, he wants to keep practicing law at HHM. If Chuck truly wanted option 1 or 2, then there is no reason for him to suggest option 3, which is a much nicer and softer option by comparison.
Chuck is manipulating HHM. He knows they cannot choose option 1 or 2 because no one wants the company to close. He is trying to force the board's hand to choose option 3, which is what Chuck really wants.
But Howard is not an idiot. He knows that's what Chuck wants. So he finds a way to make option 2 (paying Chuck) possible, which catches Chuck by surprise.
Howard saying "you win" was a facetious statement. Chuck was pretending that he wanted a payout; and Howard facetiously plays along with Chuck's lie and therefore states that Chuck "wins" (i.e. Chuck gets the thing that he claims he wants).
This is further proven by Howard's public farewell to Chuck. You can see how much Chuck does not want this to happen (but he takes it in stride, to not lose face). Howard makes Chuck's retirement public, so that it can never be taken back. Howard basically salted the earth, making option 3 impossible (or at least incredibly unlikely) to ever happen.
Chuck lost. He lost the career he loves (and remember that he loves the law more than his own brother, so this is a big loss for Chuck).
He manages to finally discard his brother away by telling him the truth, "you've never mattered all that much to me".
I don't like Chuck as a brother to Jimmy. I really, really hate him. He's cold, manipulative, selfish, and has a neverending supply of arrrogance to him.
However, I do think that his statement to Jimmy is a lie; or at the very least an overstatement.
There are a few reasons for Chuck to lie about this:
- It's an emotional overstatement because Chuck is upset. This could be due to Jimmy, or due to an external factor like losing his job.
- It's meant to hurt Jimmy; because Chuck feels that the war between him and Jimmy is not over yet.
- It's meant to make Jimmy go away; because Chuck does not want to associate with Jimmy anymore.
There is one other (bittersweet) possibility though:
- Chuck intentionally paints himself as the bad guy; so that Jimmy won't feel guilty (after Chuck's suicide) about not making amends (before Chuck's suicide).
It's the classic tale of wanting to break up with a girl. If you let her down gently ("it's not you, it's me" or "I just don't think I'm ready"); then you allow for the possibility that she still loves you after you break up, and it will negatively affect her life (not moving on, being unhappy, trying to give you a second chance, ...).
However, if you let her see you as the villain (e.g. "I never loved you anyway" or "I'm in love with someone better"), then she will not cherish any hope about a second chance (nor will she want to give you a second chance), which means that she blames you for the failed relationship; but at least she moves on with her life.
Without a clear villain, people can resort to blaming themselves. But when there's a villain, the blame clearly lies with the villain.
I think this fits Chuck's character well.
- It's manipulative, which is Chuck's favorite approach.
- Although the initial statement will hurt Jimmy, it will be less excruciating than what Jimmy will go through if Chuck kills himself while Jimmy still hopes to make amends with Chuck. That's a very pragmatic (and unemotional) approach, which again is what Chuck tends to favor.
- Jimmy was there to make amends between him and Chuck. If Chuck's suicide was already set in stone, then it would be cruel to make Jimmy think that him and Chuck will be friends again; only to then kill himself and destroy Jimmy emotionally. In a weird sense, Chuck's hurtful statement hurts Jimmy less.
He is recovering from his illness about believing he is EHS.
Chuck's illness is a big part of his life story (as related to BCS). So let me try to be short in painting the scene:
- Chuck's illness is imaginary.
- Chuck truly experiences the illness.
- Even when contemplating that it is in his head (which is what he does after Jimmy's hearing when he carried the battery); Chuck still experiences it. To the best of his cognitive abilities, he can't avoid suffering from his imaginary illness.
Although Chuck has been selfish and preoccupied with himself for most of the show; he shows some humanity when he discusses his illness with the doctor. When discussing the possibility of his illness being in his head; Chuck realizes what that means about how he has behaved himself (especially towards Jimmy).
From the script, emphasis mine:
Doctor: [Inhales sharply, clears throat] What made it so important to talk to me that night?
Chuck: [Sighs] I'd had an incident. A very public incident. Perhaps the worst experience of my life.
It was proven to me, in public, and beyond the shadow of a doubt, that there was a battery, a fully charged battery, almost next to my skin for the better part of two hours. And I felt nothing.
Doctor: What does that mean to you?
Chuck: This condition to me, it's as real as that chair. It's as real as this house. It's as real as you.
But what if it's not? What if it's all in my head? And if that's true, if it's not real, then what have I done? [My addition: Chuck gets exceptionally emotional at the thought of this]
Although he has not conclusively proven this yet; assuming Chuck's illness is imaginary, Chuck is starting to realize what he has put Jimmy through.
In the past, he was okay with asking for Jimmy's help, to the point where Jimmy was pretty much Chuck's caretaker; because Chuck had no other options, he was sick and incapable of helping himself.
But if Chuck acknowledges that his illness is self-invented; that also means that he has no justification for what he asked of Jimmy (while still being kind of a dick to Jimmy at the same time).
You are right that Chuck is starting to realize that his illness is imagined. However, with that realization also comes the burden of guilt for what he has put Jimmy through, for something that turns out to be fake.
This guilt only adds to Chuck's feeling of helplessness.