How are aware you of your pelvic floor muslces? These are often called the PC muscles, but it would be more correct to call them BC muscles for males. These are the muscles that you would use to hold in urine, or defecation even as there are more than just one muscle. Strengthening them can also affect erection rigidity.
Many men, including myself find these muscles trigger ejaculation. I too could receive oral (except by the really talented) and handjobs and not ejaculate. However, I can contract these muscles and trigger ejaculation. I believe I read somewhere that it presses on the prostate which triggers emission. I also use these muscles when masturbating to reach orgasm. Contracting them I can masturbate in under a minute. If I didn't I give up out of boredom. During sex they contract almost naturally because of the difficulty of keeping them relaxed from the pleasure, the movement and position.
So try to exercise these muscles. You can find many programs online. Search PC muscles or kegels. Realize that relaxing them is important too, so don't just try to hold them as long as you can.
Another thing to mention is Histamine levels. Men and women with low levels have more difficulty in achieving orgasm, while those with high levels have the opposite. A doctor can check your levels.
Edit: Although you had the problems before medication, there are still medications that could cause it. So if you still have problems, look into it.