Advice please! Extremely desperate!!

Okay so I’m 17 and I’m turning 18 in a few months and I haven’t had my first kiss yet and I really really want too but every time the opportunity presents itself I can’t do it. Last time I was out with my bf and he tried to kiss me and I pulled back before he could and left a few minutes later. I regretted it after and I still do. It’s not that I’m scared but I just can’t bring myself to do it ( mostly bc I don’t know how ). I just need some advice on like how to get over my anxiety about this. Because I really want to and I hate feeling so incompetent.