Does it count as sexual abuse if it was by another child?!

I had depression before, but due to family problems. But when I was younger, I was slightly anti-social and wasn't very empathetic. My parents fought a lot and my mom used to get angry really easily and hit me, slap me, or kick me. I even remember crying under the table. She often gave me threats like peeling my skin off. But I know she loves me.

She complained about my dad a lot and made me believe that he was the bad guy that would abandon me. I was already suicidal when I was 8 years old.

I was always the weird kid. I had/have friends though. I had/have very low self esteem. After moving from countries, I developed social anxiety and due to depression, I was suicidal and self-harming. Later, I had an eating disorder (this was last year) and now I'm a completely different person from 2 years ago. I'm extroverted, very friendly, and very like-able. However, it's mainly because I try my hardest to "fake" everything, the emotions and everything. But I'm actually quite detached.
I was diagnosed with PTSD last year when I was at a very low point. I was self-harming, had explosive emotions and wanted to hurt things.

Now about the sexual abuse thing...I haven’t told anyone and I won’t. It’s too embarrassing and I feel ashamed of it. And I certainly don’t want to bring it up because it was such a long time ago.
I don't really remember it. But when I was 4, the next door neighbor was a little boy around my age (maybe one or 2 years older?). We were friends and he even gave me a necklace. I remember when we simply had fun, like playing games

I don't remember how it started but I just remember very clearly some scenes where he would tell me to take my pants off and he would put his thing in my behind. We didn't have sex or anything, and his thing was never inside me, but he would just squeeze it there and move his body. He also liked to create a beat like drums when he did it.

I just remember that where he would turn off the lights and we were in the closet.
I also remember when I was in his room and he was on top of me from behind. His mom entered, but then just closed the door again and left. Maybe she thought we were just playing.

I also remember when he told me to not to tell anyone.

Once my mom opened the closet and found us in the dark. She asked what we were doing and I replied we were playing mommy, daddy, and baby. He didn’t have time to zip up his pants and I remember later my mom asked me if I took my pants off and I said no. Because I felt ashamed and knew it was wrong.

I knew there are many other times we did it but I don’t remember any more.

Come to think of it, it really impacted my life without knowing. I was very sexual at a young age. When I was 7 (3 years later and moved to a new place) I even had a high fever and I saw myself (the image of myself wearing that necklace, wearing the same clothes before I moved) and screamed “It wasn’t me. It was her/him (?)”.

Before when his picture popped up in the albums when I’m browsing the photos, I pretend that I don’t remember him. And I felt disturbed just looking at him.

Even until now, somehow I always feel exposed or humiliated when I am around boys/men who are dominating. But at the same time, I am attracted to it.

I hate him so so much but at the same time I...he was just a kid as well.

I feel so ashamed of it now. It’s been so many years and why am I thinking of it now? Did it count as sexual abuse if it was by another child? And what if I let it happen? Or even enjoyed it??