Have you read back through the postings here to find the explanation of your situation (and many other things which you probably didn't realise)? You really should.
Certainly, you do want your foreskin to operate properly, to pull back and move freely during sexual intercourse - which I presume from your observations, you have not experienced yet though you certainly might have
- whilst it is probably nicer
if it does so, it is certainly not essential
for it to do so, for you to have intercourse.
And of course, you already know
that it is not at all necessary - in fact, it could almost be a disadvantage - for you to have your foreskin pull back to masturbate with perfect satisfaction.
question is - given
that you are 18, that you have been masturbating for some six years or so, why is it suddenly necessary now
that you "have to pull back my foreskin to wash the head"? That's what puzzles me. What is it that happened? I'm really curious about this!