At 18 you should be able to pull your foreskin back AND forward with no restriction when soft. If you have a long foreskin that covers the glans when you are erect (and many men do have) then you should be able to easily pull it back off the glans when you are enjoying your penis, either alone or with others.

Five and a half inches is right on average size. Half the men in the world have bigger docks and half have smaller. So don't obsess on your size. Besides, there is nothing you can do about it. It's not how big your dick is that matters, but how you use it.

The technical name for a dick head is "glans", not "glacй".

I'm circumcised so I'll leave it to others with more experience with foreskins to answer your foreskin question.
--------------------------------------- added after 2 minutes

Whoops, a typo slipped in. It should have been "bigger dicks" not "bigger docks".