First, I am not a doctor and would recommend you see one if you can't get past this issue, but if you have had no problem in the past having intercourse with your GF and now you don't then it may well be psychological. I mean if the poor girl has gone through a 4 month period, geez just having a regular monthly knock the tar out of me, then I can see how you may feel that you could hurt her and that if you hurt her it may cause her to not like you. These are thoughts that you have to resolve and put in there place. She loves you, she had a very bad situation for which she has hopefully recovered and now she wants to be intimate with you. Express to her your love, affection and tell her that you don't want to hurt her and that she should tell you if penetration does start to hurt. That way, if and when it happens (if it does) that you are causing her pain or discomfort, she will not think badly of you, but will think quite the opposite that you expressed your feelings and how much you care for her.

I hope this helps you work it out,

Wendy

If you can get it hard while masturbating alone, but not with your gf, its most probebly just a psychological issue, if not certain. You might want to play around before penetrating and get it real hard with the foreplay.