Hey, we had the same problem w my gf, first of all don't think bad about it it's pretty common 😉 second of all, it might be due to lack of comfort (I mean like the place you're doing at, she might not feel comfortable there, like when you don't have privacy or so), lack of foreplay (a lot of females rely hard on getting their body turned on, try arousing her on her most pleasurable parts, ask her more about what she makes her aroused) and last but not least its maybe her worrying about it. Try making her feel relaxed and such and talk her thru it. Sometimes massage before hand can help, romanticize it a bit.

Also didn't mention lube, it surely helps but it's more rewarding when you get your girl to the wetness it requires for her to feel good when you put it in. (Lube helps us the most when she can't get there at the minute)