Hi Mc,
You're NORMAL.
Up to about 16 or so it is very common for a male to not be able to fully retract their foreskin when erect. The "ring" is a sign of phimosis, the tight foreskin.
You can treat it on your own. Get an erection and pull down on the shaft skin so that the glans (head) of your penis spreads the opening OR use your fingers to spread the opening. The more you do it the sooner you'll be able to fully retract.
You are close Mc, very close.
Now, not to scare you but something you need to know. If it ever does get stuck, it's called paraphimosis. It is an emergency. It can cause the tissues of your penis to die from lack of blood flow - causing gangrene or amputation.
So get stretching!!!
As to the head being sensitive, it's normal too. The foreskin has protected all the nerves and they aren't used to being touched. As you expose the head more, it will become less sensitive.
Good luck.