I don’t want to have sex with my girlfriend

My girlfriend makes it clear how much she wants to have sex while on our trip together soon. She told me that her friend wishes us hooking up a lot (could just be that she wants to). And also tells me how much she’s looking forward towards it. And I just don’t want to do it. I’m tired of living in sin. But she knows about my past. I used to sleep around before I was saved from god. And I don’t want to live that life again. But if I refuse, I’m scared of making her feel bad about herself. She already suffers from self esteem issues. And if I don’t do it with her, she’ll probably think of herself as lower than those past women I was with. However, it’s the opposite. I value her so much, that I want to live a good life under Christ with her.