21 and trouble maintaining erection without constant foreplay, even during porn and masturbation. Heart checked, hormone levels fine. Stress?

Most likely. In young men, erectile dysfunction is most likely due to stress from various sources, whether social, relationship, school/academics, financial, cultural, religious etc. You've already completed evaluation for middle age guy so unless you're taking more than just Anusol, it's most likely from stress, unless you smoke, in which case, you should quit, or drink to excess (same recommendation).