I have the same issue/s as you although I have been aware of them for many years. I have done much research and I'm currently on the same journey as you trying to fix this issue. I'll explain the problem which is very common among young men like ourselves.
Many people call this 'phimosis' although this is not technically true, as for phimosis you wouldn't be able to retract the foreskin at all when flaccid. Simply, you have a tight foreskin. The head of the penis is extremely sensitive, in fact the most sensitive part of the male body... the fact that you have never been able to expose it makes it hypersensitive to the touch - perfectly normal.
It's quite common to hear about 'stretching exercises' on the internet, where it's said you should pull back the foreskin until the point of mild discomfort, and hold it there for as long as you have patience for, to allow the skin to grow. I would definitely look up these exercises and try them, although I would advise you to take them with a pinch of salt - I tried them myself for quite a long long time and saw no difference whatsoever. If you go to your GP/Doctor they may prescribe you a steroid cream which basically thins the skin, this supposedly makes the exercises more effective, again, it didn't work for me as my condition was just too severe.
I should point out that you may have a tight frenulum, this is known as a frenulum breve (the frenulum is the 'banjo string' or the band of skin that connects the head of the penis to the foreskin) You will know if this is an issue by retracting your foreskin while flaccid, and looking at the back of the penis (the bit furthest away from you), and seeing if there is any sign of severe tightness in this area. It may look like the head is being bent forwards a little. If this is the case, the frenulum may be too tight to allow retraction when erect. It's common to have both a tight foreskin and frenulum breve at the same time, like I did.
If you try steroid creams and they don't work, your GP will probably refer you to a specialist who will explore surgical treatment - do not let them talk you into circumcision! I'll let you do the research yourself, but it's really the last thing you need, at least when you have other possibilities to explore that let you keep your foreskin, which is a very important part of the penis. I was offered a preputioplasty to loosen the phimotic ring (this is the constricting band in the foreskin that makes it difficult to retract) and a frenuloplasty to lengthen the frenulum so that it didn't restrict retraction either. I had both of these surgeries done in one go under general anaesthetic 3 weeks ago. The surgery itself was painless, I was only under for less than half an hour, and I only had a week of some discomfort afterwards. Since this I can definitely see things improving, but I think I may have to try to stretch it a fair bit more in order to achieve the full result (of being able to fully retract my foreskin while erect).
Finally, DO NOT retract the foreskin past the head before getting an erection! You increase the chances of a paraphimosis, where the foreskin is so tight it restricts blood/oxygen flow to the head. It can also start to swell making it more difficult to bring the foreskin back up. Often it can be a medical emergency, at least if there isn't any success in returning the foreskin to it's initial position. I don't mean to scare you, I'm sure it wouldn't happen but it's a risk I would rather you didn't face!
Hopefully this has helped you to understand your problem, what you can do about it, and also made you feel less alone about it - I can assure you I know how much of a pain it is. I wish you the best of luck...
First step though, go to your GP - don't be shy! Do it.
Matthew (17 years old)