Okay, now how to make this interesting...

I highly suggest you read Anne Bishop's Black Jewel Trilogy. Awesome read, and a bunch of her characters deal with this type of problem. She calls it "entering the twisted kingdom," which corresponds with losing their mind. One of the MCs has to come back from this, and his journey is pretty detailed throughout the series. You might like it, and it might give you some ideas.

Good lord I hope something here helps you.

So your MC has large portions of his backstory/motivations missing? Well. As far as motivation goes you can always sap it from him by an exterior source. He can be indentured, enslaved, stuck in debt up to the point that he HAS to put up with the situation he's in, day in and day out, or else his little old mother won't her her cough medicine and die.

You can make him emotionally distant and distraught. They were abused physically/sexually as a kid. Can't connect with someone and, whenever something overwhelms them, they dissociates immediately.

Hell, you can just make them a stoic.

You can have them know fully well about their past, but bring out that they don't look out and consider it or talk about through dialog.

"Anyway, enough about me! I've been going on about my sister for ten minutes. What about you? Do you have any siblings?"
"I'm sorry, I've really got to get back to work."

Or you could do something much more subtle. Sure, your MC is self-aware and remembers everything. Maybe, piece by piece, they discover what their actions and thoughts have really been. Clarity through self examination. Such as: The MC knows fully well about what happened to them in a fight. They were there. They remember it. What they don't get (until, at least, after the book starts) is how what they said and did hurt the other person. Or how their actions after the breakup caused a series of positive (or negative, whichever way you want to swing) actions in their own and others lives. The experience is there but the wisdom is missing.

Oh well. Hope something helps.

make his character come out through responses to outside stimuli. I you make his experience highly perceptual, rather than cognitive, we will get a sense of what sort of a person he is. Otherwise I do not know. The film 'Dark City' might help you to get a sense of how that can work.

I agree with the above posts.

Your character has no choice except become totally SUBJECTIVE.

If someone doesn't have enough power or wherewithal to be OBJECTIVE and take charge of a situation, then only the unfolding events can possibly guide the character to the next step.

Think of your character in the third person (if you're not doing so already) and ask "what would happen if?" to a series of actions in your plot. Yuo may find some interesting answers!

J