You have trouble coming, it takes a long time, unless it's a quicker. You both become very unsure of it. This phenomenon is more common in men, although there is not much talk or writing about it. It often has to do with insufficient concentration on your own horny feelings. You remain, as it were, a spectator of yourself, of the lovemaking. It should also be done more or less your way: does your girlfriend like it, does she get excited enough, does she like it, does she cum?
You are too concerned with her and the lovemaking. As if you are trying to control it, as if you are in control. Is it important for you to keep an overview? That you like to keep things under control? Is that something that suits you, also in everyday life? There is nothing wrong with that, which is a characteristic that many people have, but during a lovemaking it can get in the way. After all, cumming is getting out of the world for a while, letting you go, losing control. That is difficult for some men (and women!), They are unconsciously afraid of it and so the sex and especially the orgasm become quite convulsive: it has to be done in a certain way, you must not be distracted, it must not move. That can be quite annoying for your girlfriend.
A tip from Hotlinks; make sex a bit more playful, not according to the fixed pattern of arousal, getting horny, having to come. Relax, and let it go, and don't forget to enjoy what is there and what you feel. You will experience that things will slowly improve.