Try stretching the opening using your fingers to spread it gently OR pull down on the shaft skin while you have an erection.j You don't have to hurt yourself just do it as often and as far as you can. The skin and the blood vessel WILL stretch in time.
Otherwise, see a urologist. They can determine if there is a problem and how to treat it.
Perhaps you are talking about your frenulum.
That piece of skin at the bottom of the glans is called the frenulum. It is normal and it is supposed to be there. The frenulum serves to limit the rearward travel of the foreskin. The frenulum has a high level of sensation.
If your foreskin bunches up behind the head when you retract you most likely have a short frenulum (frenulum breve). Test it by pulling back on the foreskin. If the head of your penis bends down when you pull back your foreskin, you have frenulum breve.
You can stretch the frenulum manually just like you do your foreskin. That is the best course of action if the frenulum is too short to permit your foreskin to retract. Search the Internet for:
“How to Fix Phimosis and Tight Foreskins, Solutions That Work”
and you will find the information you need.
There also is a minor operation to relieve the frenulum, called frenuloplasty and another called frenectomy. One of these might be necessary, if the stretching does not work. Urologists do this.
Don’t let anyone sell you a circumcision. Circumcision is NOT necessary to treat this condition and is very harmful.