I 20/M can't stop thinking about cheating on my 19/F girlfriend. We have been together since Senior Year of High School. I don't want to and I'm really scared. Help

I 20(M) have been with my 19(F) girlfriend since the senior year of high school. I feel really guilty because of the thoughts that I have been having lately. These annoying thoughts that creep into my head, about us really not having much shared first experiences. I know it’s really stupid, and I feel really guilty about these thoughts because it’s not her fault that we couldn't have those experiences. It makes me feel even worse because I have no reason to be thinking this. The first time we had sex, I remember it vividly. We had known each other for almost a year, and one day, when we were talking, she brought up that she had never been on a date before, and neither had I. So we decided to lose out on “dating virginity together”. I spent two months planning it out. She’s a big animal lover, so I planned our date at the zoo. I even paid for a special animal meet-up and playing, she could stop talking about how much fun she had on our way back. But, to finish the date, I had gotten a hotel room, put candles everywhere, and even some non-alcoholic wine. We actually made love that day, and she said something with more context that makes a lot more sense. We were walking her back to her house, she said, “I wish things had been different, I really would have liked you to be my time.” I didn't fully understand the weight of those words until much later.

She was my first everything: first kiss, first relationship, first time having sex, first date, all the firsts you can think of. I, however, haven’t been her first in much of anything except a date. She has been through multiple traumas throughout her life, both sexual and emotional, when she was young, which led her to be a bit promiscuous, which I'm not going to get into because she told me those things in confidence, and aside from her therapist, I'm the only other person she has ever told these things to. So, it feels wrong for me to cross that line. The important part is when she finally told me all the trauma she had, I suggested not having sex anymore, because I thought maybe she hadn’t dealt with her trauma to the level where she should be having sex, and this would be a step to her healing. It took a lot of convincing and talking for me to get her to agree. In her words, she said she would be burdening me with things that are her fault and her messes to deal with when I haven’t done anything. Which only reinforced my idea to see how much this trauma was really sticking with her. That's when the thoughts of our lack of first experiences really started coming out more.

Now those thoughts are just becoming more common. And I know it’s awful, but sometimes a thought comes that maybe, if I have sex with another person, this feeling of a lack of first experiences would go away because it wouldn’t be just my first experience anymore. It’s a stupid thing to think, I know. It’s not even like I have any person in mind; it’s just this feeling that if I do it, then maybe the feeling will go away. I really love this girl, and the last thing I want to do is hurt her in any way. I especially don't want to stop all the progress she's been making. I'm scared I might add to her trauma. Like all these things I've helped her do will be poisoned because every time she thinks of them, she thinks of me and avoids doing them because I would have hurt her. If that makes sense. After she told me about all the stuff that happened to her, I was the one who pushed her to get a therapist. I was the one who said we shouldn’t have sex until she feels more at peace with it. And she’s been thriving, finally getting all these things outside with someone who can help her, and the last thing I want to do is hamper her in any way. Am I allowed to feel this way?

TLDR- don’t feel like my girlfriend and I have a lot of “First” experiences, and unwanted thoughts are making me think I should have sex with someone else. Help.