Should I tell my girlfriend about my worst mistake(s)? Or should I take it to my grave?

Throwaway for soon to be obvious reasons. A little over 9 years ago I was going through puberty and my hormones were all over the place (I'm male) and I had taken to watching porn. I didn't know what j was doing all I knew is that I was really desperate. I had gotten into facesitting and that was pretty much all I watched. Long story short, I had been going around and asking a lot of girls my age to sit on my face. Most said no, so I took to tricking them. I would ask them to get on their knees facing away and then lay down behind them before asking them to sit back. I had done this with, I don't even remember how many. One of them being my sister. My sister didn't see anything wrong with it, her being 2 years younger. It went on for a little while, not sure how long, when I branched out and asked my mom. She, however, did see something wrong with it and talked to me about it and (I presumer) talked to my sister about it as well. I stopped with my sister after that. I didn't stop with other people though. I had maybe only done it with 3 others, if even that many, when one of them had told her mom and her mom called the police. Skipping ahead, I wasn't arrested but I did have to appear in court at 12 years old. Judge sent me to a remedial therapy program and my record was sealed. Legally, as far as I know, I can say that I never went to court so that is not an issue here. My only question is do I tell my girlfriend of just over 11 months. She is the love of my life, the greatest thing to ever happen to me. She is my soul mate and the woman I will marry one day. I'm so so terrified beyond anything else on this planet of losing her but I also don't want to have this secret hidden away. Realistically I don't see it ever coming to light but you can never truly know. My sister has referred to it as me SA'ing her and while I wish she wouldn't I also don't blame her in the slightest. My brain has just kind of gone on a runaway anxiety train today and I'm worried she will one day tell people or hold it over my head in some way. As I'm typing this out I feel as though I've answered my own question but I feel as though I should still get others advice on this. Normally I would confide in my girlfriend but for obvious reasons I can't. Typing this out I have realized just how heinous what I did was but I want to mention that I was 12 at the time and all the girls were right around my age. I also haven't done anything remotely (remotely doesn't even feel strong enough for this, more so like absolutely nothing at all). If this is too dark or serious for this sub reddit I will take it down and probably ask an actual therapist or someone else like that. I'm just hoping to get an answer with some degree of anonymity