As one brother to another, I urge you not to overreact to your recent episode. Here's what I suggest:
Prior to your next session, eat very lightly, avoiding fatty food, even focusing on fruit and vegetables. You'll want to eat afterward, not before.
Get some exercise throughout the week - simply walking or riding a bike helps your circulation which, of course, helps blood supply to the twig and berries.
If you are just starting to use an ED prescription, start with the lowest dose. You seem to indicate that you got the prescription for 20mg doses, which is the max. If you take 20's, I predict your heart will race, your ears will ring and your head will turn bright scarlet from the blood pumping in your system.
Try getting a pill cutter ( Dollar Tree for a buck) and cut your pills into quarters. I bet the 5mg quarter you cut for yourself will be adequate, and avoid the dramatic physical effects. Nothing wrong with the Rx for 20 mg - by cutting them you get 4x for your money.
Don't swallow the pill when you decide to take it. Reaction time in the gut is hard to figure. The pill is bitter, but simply pop it under your tongue as you are about to reach the date. The medicine will absorb very quickly into your bloodstream through the thin membrane under your tongue.
You may want to try Maca Man, an all-natural supplement from GNC, in addition to your prescription. Take according to the label, 3 pills, 2 to 6 hours before sexual activity to help your circulatory system.
Finally, hobby time is fun time. Plan your dates when life is less hectic and other matters are not cluttering your mind. A little pre-game pep talk helps, to realize how amazing life is and how awesome the act of physical interaction between a beautiful woman and yourself can be. A feeling of gratitude helps the attitude.
These are all steps I take before every sacred date...Good luck