Can't keep it hard

I'm 21. As time goes on and I get older it seems its harder to keep it hard. I think its better explained with examples.

If I've been doing it 10 times a day, the next day I'll be able to keep it hard just the first time maybe and even then not for long. The next times will be grudually softer, plus it starts to feel numb and more numb. Ejaculating gets harder, but eventually comes, even if its like as soft as if it wasnt erect at all.

If nothing has even touched it except my underwear for say 2-3 days(anymore doesn't make a big difference) I get hard like hell especially the first time. But even that day it will go softer gradually, just will take longer. Say the 6th time will be as soft as the 3rd time would be in the first example.

I assume this is somewhat normal to a point but, I don't know. It seems I can only stay hard properly the first couple times if I'm doing it every day. It's really annoying cause I want more. I don't even get why nature would make it work like that in the first place anyway but whatever.

Any suggestions?

I don't think its psychological cause like I said in the 2nd example it gets hard all fine at the start. Maybe I just need more stimulation later on... but at first I am fine.

Oh I also exercize, I do weightlifting(dont get any testosterone supplements or any kind of supplements that would affect erection), I generally feel pretty manly so I don't think I might have low testosterone but I might consider going to test out.

I also definitely don't feel any performance pressure or anything. At least I don't think so.