Erection isn't that hard. Erection?

Hi doctor, I'm a 21 y/o male. I have a series of questions. I will organize them as follows:- 1)I can masturbate smoothly and I am able to get an erection while masturbation. The only thing is, I feel like the erection which I have in my sleep is much harder than the one I get while masturbating. I feel like I should have that kind of erection when I am masturbating, which isn't the case. I feel that when I wake up and I am having an erection. Is there something wrong? 2) I read a lot of articles - but still would love to hear it straight from a doctor. Does excessive masturbation lead to impotence/ED? 3) My foreskin wasn't able to roll back until I was 12 or 13 y/o. Even now, when the penis is completely erect, I have some pain in rolling the foreskin over the penis head fully. Should I roll it back before erection? I tried that.. it leads to difficulty in getting it infront after the erection is lost. Your valuable advice and expertise is certainly appreciated! Thank you!