I'm 22 and it's hard for me to keep an erection without constant stimulation.?

Viagra (sildenafil) trial ? Performance anxiety could be responsible. A trial of viagra (sildenafil) or another phosphodiesterase inhibitor could allow for the fear to be overcome. After 2-6 sessions as such it would likely no longer be needed.
That's unusual. That is unusual for 22. It makes me wonder if you are on some medication which is creating side-effects? If so, i would discuss with the dr. Who prescribed. Otherwise, my suggestion is to get checked out by a urologist. Sorry i can't tell you more without knowing more about the situation.
Ed. It could be psychological , organic (low testosterone, diabetes, vascular disease) due to pelvic surgeries get examined.