Inexperianced - is this sort of thing common?

The more you concentrate your efforts on resolving this issue, the more of a problem it might become. Your gf sounds mature and understanding, so maybe she is the one you need to talk to. How about enjoying the intimate moments without it having to lead to anything else? Seems to me that your anxiety regarding sex is leading to more problems. Remove the cause of anxiety, then things should work out slowly and over time.