Insecurities after finally getting to be with a crush as a girlfriend?

I don't really know how to deal with it since it's all new territory for me tbh. I'm a virgin still in my mid-20s and have never really had a real relationship before so I just don't know exactly how to deal with everything but want to for her.
I had a crush on her for like years. 4 years since we first met but she was taken. I spent a long time trying to find someone else and hearing how we'd never end up together and genuinely knowing it even though my fantasies kept up and I had to listen to her talk about how great the sex is while I just wondered what sex is even like and if it'd ever be like that if we got together.
Long story short, yeah, we got together very recently. And I just have those moments in my head. When we get close and touch each other and I just can't help but think how I was the guy on the sidelines for her the whole time thinking of her and she was with him fucking him and telling me about it, I mean, as friends do, I know it's not her fault it's entirely my thing but it still just sticks with me. Like I just don't know how I don't feel like I'm always number two or five or ten.
She's older and has had a few partners before me and that bothers me a bit, don't crucify me for it, but it's just all the times I had to listen to her sex stories with him when I was pining for her that really get to me. It really sucks too because now that I'm privy to things I get to hear how I was actually one of the only legitimate sources of support for her as that relationship was falling apart during that time after years together and how she really did like me and wanted me and she really works to make me feel wanted knowing my self-esteem issues. I only just recently started getting success in my career too so I guess it's just hard to trust this is real. What do I do?
Insecurities after finally getting to be with a crush as a girlfriend?
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