I don't remember the bad (usually) either.
I did start writing things down at the end, so I wouldn't forget them (and then believe I did the wrong thing), but I try not to dig through it.
The only thing I have right now that shows me how upsetting most of the relationship was are the poems I wrote. They're all really dark, and some of them look like I was about to... Well, you can guess.
I don't remember what inspired me to write any of them, but they're there, and that's how I was feeling.
I think that's the best way for me to remember it, rather than start crying about specific incidents I'd rather just look back and say "I was really upset most of the time. I'm really happy most of the time now. I did the right thing."