Rock hard erection on porn but not that hard when masturbating?

So I've decided to quit porn since it doesn't bring me any satisfaction or benefit. I do not think I'm a hard porn user but I might addicted because I observed that I have rock hard (not metaphorically but literally) erection only while on porn.
*I use porn since 14, I'm 22 now. I used to do it daily.
I lost my virginity at 18 y.o. but the girls I had sex with (not many anyways) liked it a lot but I was never satisfied with my erection...
If I do masturbate, my erection is not really hard. After a period of using porn, I barely have an erection.
Anyway, right now, I'm barely using porn and my erection improved but I cannot understand why is it so hard on porn but not really that hard without it? It's hard enough for penetration but not like on porn..
I masturbate every other day, sometimes daily, with or without porn and maybe I'm masturbating too often?
I am a bit afraid of this since I have a girlfriend, for the moment she's abroad but in 2 weeks she's back here and I'm afraid I won't be able to satisfy her (and myself). We didn't have sex yet and I wouldn't want to disappoint her.
Any help appreciated.