From what you have said, I do not think you have a problem with your foreskin.
I find that condoms de-sensitise the head of my penis during sex, too, but have not had "great difficulty ejaculating" wearing a condom. In fact, this desensitising effect of the condom has probably allowed me to last longer. You should not put a condom on over an unretracted foreskin.
I don't think your frenulum is too tight, particularly if your foreskin retracts automatically during foreplay. My foreskin rests just behind the head when my penis is erect, too, so I'm sure your foreskin is not cutting off blood supply to the head.
Perhaps you are a bit too anxious about this. I know it's easy for me to say, "Try to relax". However, maybe with the passage of time, when you've had more experience, and when you feel more comfortable, things will improve.
__________________ I hear the beat of a different drummer