Have no difficulty reaching orgasm via masturbation, but great difficulty via intercourse or stimulation by another person. any ideas to resolve?

Missing the big "O": what are you doing to yourself that your partner isn't?? - may be related to not enough stimulation of your penis during sex. Try anyone of the lubricants marketed to enhance pleasure. foreplay may also help. Is there a concern on your part for unintended pregnancy - it may also be a mental "block" issue. Try different positions as well good luck.