I hear how tormented you are. Your problem can be resolved, so don't despair. From a purely mechanical point of view, premature ejaculation is one of the most easy problems to solve.
Most men who ejaculate too quickly do so because they haven't learned how to recognize their premonitory sensations (the feeling just before the point of no return).
A qualified sex therapist can show you how to recognize these sensations. Then, the therapist can teach you specific techniques for delaying orgasm. These techniques include the Squeeze Technique and the Stop-Start technique, which are fully outlined in my Advice Archives. Just do a search to find out more.
Because you haven't had the right help, the problem has magnified to the point that you are becoming scared 'unstiff.'What I mean by this is that the chemicals that flow in the body when a man is frightened are incompatible with the chemicals that need to flow in order to achieve and maintain an erection
So, please, take a chill pill. Next, you and your girlfriend, who sounds totally supportive, should do the two techniques I described above together. View the exercises as the most fun homework assignment you ever had. When you do the exercises, talk to yourself and take all pressure off.
Remind yourself that you are learning a much needed new skill. This isn't very different than learning any other new skill. It takes patience and practice.
While I am sure that you have a skills deficit, we need to also make sure that emotional issues aren't adding fuel to the fire. Are you sure that you really want to have a relationship? I know that you do consciously? However, the unconscious part of your mind may feel differently. You yourself said that no woman would want to stay with a man who can't delay his orgasm. Is it possible that you unconsciously don't want a woman to stay with you? Are you afraid of closeness and intimacy? Are you scared to get close and be rejected or abandoned
If this is true, then realize that the unconscious part of your mind can actually manufacture any number of problems, including premature ejaculation, in order to drive any potential partner away from you.
So, by all means, make sure that there isn't an emotional issue rearing its head (no pun intended). Boning up on your skills and looking into your emotional issues is your winning ticket.
Let me know how you make out.