I can relate to those thoughts and I've just been diagnosed with heavy depression. If you're also tired all of the time you might want to go to the doctor's.

Anyway regarding your parents: Who they are now might not have been who they were when they decided to have you (I know mine weren't).
Either way, you're not responsible for your parents happiness, but I'm pretty sure they're very happy that you exist, they just don't like each other.

It sounds like you're taking on a responsibility that isn't really supposed to be yours, so I just want to say: You didn't keep them together, their unwillingness to accept that they should be apart did. They might have used you as an excuse for this unwillingness, but that's on them, not you. You are not responsible for that, just like you're not responsible for their relationship. They are adult sapient beings who should be able to look at their relationship and realise that they are bad for each other. The fact that they have a child together shouldn't be a complication, in fact it should have made it easier for them to make the responsible decision, so they could provide you with two stable loving homes, instead of one unstable home.
You are not responsible for your parents. They are responsible for themselves and in fact they are responsible for you, because they are your parents. It should not be you lamenting your existence, but them lamenting the fact that they have pushed you to think such a thing. You are not responsible for your parents.