Hoo boy do I know about horrific, intrusive, mind boggling guilt. I've got some things in my past too that are tucked away so deep that they only come up to bother me every few years. During those times I feel suicidal, panicking that I'm going to Hell for what I did and the potential damage I wrought on other people. Imagined or not... Eventually I get so mentally exhausted that my brain buries the memory/thought again to surface another day...

But take comfort from another voice saying you're free from judgement. At least in my eyes. Normally someone with a budding sexual deviancy like pedophilia will have recurring, often disturbing thoughts toward children and teens well into their adult life. It doesn't sound like you were having sexual thoughts about your cousin, just curious ones. It also doesn't sound like you're struggling with sexual thoughts toward children as an adult, just an overwhelming guilt and depression.

Do you think it might ease your mind to talk to her about it? Depending on the ease and comfort of your relationship now, you could mention that you remember that happening and express your remorse over it. Because whether you believe in your innocence or not, your feelings are still dogging you about it. A lot of times just not calling more attention to an embarrassing faux pas or screw up is the best way to handle it, because the louder you are about it the louder everyone else will be.

But in this case it might help you more to get some reassurance that you aren't the cause of her problems. And I don't personally think you are. She very well might look back on what you two did and cringe a little, or maybe attribute it as a symptom of something else going on. But in the end you did the right thing and followed your gut instinct; stopping it when you felt uncomfortable.

I know this is rambling a lot and I'm sorry. You're gonna be okay though. <3