Two things that may help: 1. Some men find that a cock ring can help maintain an erection (put on once already erect). Bonus... some of these have fun things for your gal also... nubs, lil vibrators, etc. 2. Here's a tip that may help make condoms feel more natural for you... before you put the condom on, put a drop or two of heating lube on the frenulum area. Then apply condom & proceed as usual. The heating lube can be either the yummy, fruity stuff from a specialty shop, or as simple as the new heated KY sold practically everywhere now. The friction of intercourse will gently heat the lube (it doesn't get piping hot unless you blow on it), which some guys say creates a more natural feeling.

Smart girl you've got, insisting on a party hat for backup. My mom was on the pill 28 years ago, & oops, here I am! In any case, don't stress too much about it... nobody has a perfect, porn-quality sex life 100% of the time. Just relax & have fun!