The best course of action is to discuss this with a sex therapist/ psychosexual counsellor. I was one before I retired, so here are some comments, which may or may not be helpful.

You are labouring under a pressure to perform well. Sex should be fun, playful, a pleasure and not a duty or expectation. You need to cultivate an attitude where it doesn't matter whether you get hard or not, whether you cum or not, whether you penetrate or not. Just being with a person that you love and to whom you are attracted is what matters.

It is also important to explain to your girlfriend that it is not something she is or not doing that results in no orgasm for you. Make it clear that she is attractive, hot and sexy for you but that for you an anxiety to perform well is inhibiting your ability to cum.

Begin your physical and intimate relationship with massage. You massage her and then she massage you. If you get an erection then fine but you are not going to do anything with it together. You or she can touch it and play with it but the erection is not for having sex with or having an orgasm from. You can kiss, cuddle, caress - of course.

Once the pressure to perform is off, your subsequent massage sessions can be a little more erotically focused but cumming is still not to be seen as the end result. You may each masturbate yourself to orgasm -either together or later alone. Again kiss and cuddle to show your affection.

Next, you can bring her to orgasm with your fingers or mouth at the end of a massage exchange session.

Then, after several sessions of not seeking orgasm, she can prolong the massage of your penis with her hands and/or mouth. If you cum then great. If not, it is still not a requirement or expectation. If it helps then use fantasy to heighten your arousal.

Avoid making it a chore for her (or you) to get you off. If it isn't working within reasonable time then simply stop and move back to kissing, cuddling and caressing.

You may still need specific help from a professional consultation if the above program doesn't sort it out. The sessions above should cover about 10 to 12 separate occasions with the same girlfriend over a period of about a month to 6 weeks. It could be shorter but don't try to speed up. Trying to sort it out quicker could be another aspect of your drive to end up by cumming. You can't try to cum any more than you can try to sneeze. It happens when it happens - if it happens. The goal of sex is to share pleasure and emotional intimacy not to get your rocks off (Many men see it as the latter).

It may not apply to you, but some men cannot come because they feel resentment towards women - maybe their partner or maybe some other woman such as their mother or a teacher or a previous partner. This may need counselling.