b) you stay hard for a long time, without ejaculating, prior to penetration ... but once you penetrate ... you release almost immediately
If that's the case, I have a suggestion for you.
I'm assuming that you don't wear a condom during foreplay and oral. Correct? If I'm right, then a possible solution would be to slip on a condom (which offers some degree of desensitization) ONLY prior to penetrating.