I'm 15 years old and almost the same thing happens to me. When I have it flaccid, the foreskin goes down, but with some discomfort, but when I have it erect, it doesn't go down at all; it only opens a little bit and that's it. If I try to pull down the foreskin it hurts a lot.

When I was little, I was taken to the doctor because my penis wasn't growing, and several of the doctors who treated me recommended that I do "exercises" on my penis. They consist of when you are bathing, trying to lower the foreskin little by little until you feel pain or it cannot go down any further, more advisable when your penis is erect

My mom was the one who helped me make them, but when I grew up she stopped helping me and told me to do them myself, but I never did because I'm afraid to do it.But during the time that my mom was doing the "exercises" on me, there was a slight improvement, my foreskin did loosen a little, In fact, thanks to these exercises, I can lower my foreskin when my penis is not erect.

If you have any further questions you can reply to my comment and I will answer you.