How far does the foreskin normally retract?!

How far does the foreskin normally retract?

i have tried pulling it back over the head,
but then it feels like the skin is rolling up
and i wont be able to pull it up again.
when i get an erection, it makes it
even harder to retract my skin.
it'll usually only go halfway
before it feels like it'll tear.

also during sex, does it usually retract?
and if so, how far?

Normally the foreskin should be able to easily retract all the way back so the entire glans (penis head) can be exposed. This is true in masturbation or sex and whatnot, and some guys it retracts by itself upon erection (though not in all guys). It sounds like you've a condition called phimosis, where the foreskin opening is too tight/narrow for you to retract it that far comfortably or without pain. Fortunately there are many treatments for phimosis.

The first thing you should try are stretching exercises. In one stretching exercise, take 2 fingers (1 from each hand), "hook" each finger on either side of the opening, and stretch in opposite directions as much as possible (like increasing its radius) without pain or discomfort. Stretch and hold for a few seconds, then let it relax for a few seconds, then repeat the process for a few minutes each day for a few weeks to a month. It's best to do this right after a shower/bath when the skin's most flexible (see link 1 for more info).

If that doesn't seem to work after a month, ask a doctor about certain prescription creams. In one study, these creams are the most cost-effective treatment a doctor can provide (link 2). Be aware that these creams may also take up to a month for the full effect and resolution of phimosis, but you can speed it along by combining them with stretching exercises.

And worse comes to worst and nothing works, it might be worth considering surgical intervention (but only after the above 2 treatments have been attempted and failed). Note that surgery doesn't mean circumcision as there are other procedures that are cheaper, less painful, remove little/no foreskin, and have a quicker recovery time (link 3). Regardless, any surgery should be considered a last resort for when everything else you're willing to try has failed. Thus you should ignore any comments that suggests circumcision as the first/only line of treatment.

See links 4 and 5 for more info on how to deal with phimosis. In the end, while a circumcised penis is better than one with a foreskin that doesn't work properly, a penis with a normal functioning foreskin is preferrable to both. There are certainly potential advantages to staying uncircumcised and having a working foreskin (links 6, 7, 8) and as long as you maintain proper hygiene of the area and safe sex, you shouldn't worry about STDs.

Good luck with whatever you decide to try and I hope everything works out.