I'm a sexual abuser

Just my humble opinion, but I think you can relax and let yourself off the hook. Its not that bad, what you did. It sounds like your cousin was not much younger than you and she consented in a way, she was curious. Not that that makes is okay, but it can be normal for 10 year olds to be curious.

I think I was truly sexually abused, because it began at the age of 3, and there was some extortion involved. I.e., I did not feel okay about it, but the person manipulated me into it, and he was older and in a position of power. I think that situation was probably more damaging than what happened between you and your cousin. My abuser was also a serial abuser, he had other victims. While it sounds like with you it was kind of an isolated incident.

You sound like you have a good conscience and you wish no harm upon others. Children can be curious and sometimes act out that curiosity. What happened between you and your cousin sounds fairly benign and I honestly doubt it did any harm to her, yet it is sweet of you to be concerned. I hope you stop feeling guilty and forgive yourself.