How to stop feeling pain during sex

It could be that the initial pain that your girlfriend felt on attempted penetration has caused her to feel anxious about penetration in general. It certainly sounds this way.

The more anxious she is, the more tensed up she is likely to be and hence the more difficult and painful penetration will become.

The most obvious thing to consider would be her level of vaginal lubrication. A girl needs to be sufficiently aroused for penetration, hence the need for foreplay. If she does not produce enough natural lubrication despite foreplay then you might consider using some artifical lubrication (water based) for that specific purpose.

Penetration is likely to involve some discomfort when it is a girl's first time but it shouldn't be excrutiating under normal circumstances. Your girlfriend might consider getting a check-up to make sure that everything is as it should be down there. That would be a good starting point and might alay some fears.

Thank you. I will talk to her