Are you a giver? When you're having sex, are you constantly concerned with her being satisfied? After plowing her for awhile, do you think about her getting sore, your cock becomes desensitized and you worry about not cumming? I'm not a psychologist but I've been there before too. When you masturbate, it's all about your thoughts and doing it the way that feels best for you, that's why you can cum then.

My suggestion next time you have sex, become a little selfish, put some lube on your cock, and get lost in it for yourself. If you fantasize when you masturbate, do so when you're fucking her, maybe she'll even act them out with you. Try masturbating together, it might take the pressure off cumming inside her the next time.

Hope this helps.

This is a psychological problem,......quite probably you are uncertain about the relationships of of becoming too involved with women before you are ready to commit to one particularly.

You need to carefully sort out exactly how you feel about these partners and also come to a clear arrangelment with them as to how much commitment the relationship has and the place of sexual pleasure in this.......

......The ideal situation to get over this is for a casual pleasure relationship which both sides understand is only about sex.