She intended to kill herself, but wanted to wait for Nick to be publicly lynched in the media - but she changed her mind as she believed herself to be guiltless and thought she didn't deserve to die.
Whilst this is Movies & TV, this entire answer is composed of quotations from Gillian Flynn's original book, Gone Girl, as it is obviously the definitive source of information.
In the book, there is never any doubt that she intended to kill herself.:
...So I began to think of a different story, a better story, that
would destroy Nick for doing this to me. A story that would restore my
perfection. It would make me the hero, flawless and adored.
everyone loves the Dead Girl.
(from Chapter: Amy Elliott Dunne - The Day Of)
She later goes on to say:
But I am careful with money, my point. I have enough to live on until
I kill myself. I'm going to hide out long enough to watch Lance
Nicholas Dunne become a worldwide pariah, to watch Nick be arrested,
tried, marched off to prison, bewildered in an orange jumpsuit and
handcuffs. To watch Nick squirm and sweat and swear he is innocent and
still be stuck. Then I will travel south along the river, where I will
meet up with my body, my pretend floating Other Amy body in the Gulf
of Mexico. I will sign up for a booze cruise - something to get me out
into the deep end but nothing required identification. I will drink a
giant ice-wet shaker of gin, and I will swallow sleeping pills, and
when no one is looking, I'll drop silently over the side, my pockets
full of Virginia Woolf rocks. It requires discipline, to drown
oneself, but I have discipline in spades. My body may never be
discovered, or it may resurface weeks, months, layer - eroded to the
point that my death can't be time stamped - and I will provide a last
bit of evidence to make sure Nick is marched to the padded cross, the
prison table where he'll be pumped with poison and die.
I'd like to wait around and see him dead, but given the state of our
justice system, that may take years, and I have neither the money nor
the stamina. I'm ready to join the Hopes.
(from Chapter: Amy Elliot Dunne - Five Days Gone)
So certainly at this point, her thoughts are of suicide. They are pretty well thought out, too.
As time goes on though, she changes her mind:
I have changed plans only twice so far. The first was the gun. I was
going to get a gun and then, on the morning I disappeared, I was going
to shoot myself. Nowhere dangerous: through a calf or a wrist. I would
leave behind a bullet with my flesh and blood on it. A struggle
occurred! Amy was shot! But then I realised this was a little too macho
even for me. It would hurt for weeks, and I don't love pain (my sliced
arm feels better now, thank you very much). But I still liked the idea
of a gun. It made for a nice MacGuffin. Not Amy was shot but Amy was
scared. So I dolled myself up and went to the mall on Valentine's
Day, so I'd be remembered. I couldn't get one, but it's not a big deal
as far as changed plans go.
The other one is considerably more extreme. *I have decided I'm not
going to die.
I have the discipline to kill myself, buy can't stomach the injustice.
It's not fair that I have to die. Not really die. I don't want to.
I'm not the one who did anything wrong.*
The problem now though is money...
(from Chapter: Amy Elliot Dunne - Eight Days Gone)
Ultimately, the final turning block that cements this idea to live is the robbery she suffers at her cabin, when Greta and Jeff take all her remaining money. In the very next (very short at half a page) chapter, she comments:
I am penniless and on the run. How fucking noir...
I think of suicide
again, how a prisoner on suicide watch spends twenty-four hours a day
under lights, an awful thought. My gas tank is below the quarter mark,
an even more awful thought: I can drive only about an hour in any
direction, so I must choose the direction carefully. South is
Arkansas, north is Iowa, west is back to the Ozarks. Or I could go
east, cross the river into Illinois. Everywhere I go is the river. I'm
following it or it's following me. I know, suddenly, what I must do.
(from Chapter: Amy Elliot Dunne - Nine Days Gone)
In the very next chapter, she meets up with Desi (Neil Patrick Harris in the film) and is thus "confirmed" to be alive.
So to summarise A LOT of quoted information from the book, she certainly had a definite intention to kill herself when she started out. She waited because she wanted to see Nick be lynched in the media first.
However, after seeing her husband squirm and fight on television, a growing sense of resentment built up in her and she felt she didn't deserve to die as she was totally guiltless. Instead she chose to live. However, being robbed of all her remaining possessions and knowing she couldn't get anywhere to continue with her life, she was left with two choices - die or return. She chose to return (to Desi at least) and "live" again.