I can only masturbate when foreskin covers the head

Hi crazycrazyskull, when you say it hurts do you mean because your foreskin is tight when it's pulled back or do you just mean because the glands are sensitive when exposed and touched?

i think both

Do you find it easy to pull your foreskin back and does it get tight over the glands as you pull it back when you soft and erect?
The more information you can give the easier it will be to work out what you need to do :)

when i am soft it is easy to pull foreskin back over head no pain. When i am erect i can also pull foreskin over head however it is not comfortable and cannot keep it retracted for long periods of time feels quite tight.

It sounds like your foreskin is still a bit tight when your erect and needs a bit more stretching, the best time to do that is when you take a bath as the water will help soften the skin and make stretching a little easier. It's not something that will happen over night and will take a while before you see any difference, as for the sensitivity of the glands the more you can keep them exposed and use to being touched the less it will feel uncomfortable to be touched.
If you would like more information on how to stretch your foreskin feel free to ask.

yep no idea how to stretch foreskin please fill me in. and when i keep my penis head uncovered i seem to always get erect :/

Here are a couple different ways for you to try and if you would like more ideas just Google how to stretch my foreskin and you will even find a you tube video on it.

1. Pull Back Stretch
When you are unable to insert 2 fingers inside the foreskin, this method is the most appropriate way to move ahead.
Move the foreskin back as much as possible without leading to pain. Some discomfort is to be expected. Maintain this position for 30-40 seconds, and then bring the foreskin to the front again and relax. Repeat the same procedure 10 times in the bath as the heat can loosen the foreskin a bit.
After a few weeks of this exercise, you will be able to move the foreskin to the rim of the glans when the penis is erect. Seek medical help in case the foreskin gets trapped behind the penis glans.
2. Ridged Band Stretch
Once you are able to insert 2 fingers inside the foreskin, start practicing this method to solve the question of how to loosen foreskin. In this method, first find the phimotic ring in your foreskin with two fingers, the part which is difficult to move over the glans, and pull in opposite direction and hold for 30 seconds without causing pain. Some discomfort is to be expected.
Repeat this process 10 times a day for few weeks in the bath and spray the exposed head of penis with tap water or take a shower to reduce the sensitivity. Do not give up too early if you want to see the results.

Good luck and if you have any concerns or questions feel free to ask.