Masturbating in the wrong way is the main cause of phimosis.
See this scientific study that links masturbation to phimosis: http://www.cirp.org/library/treatment/phimosis/beauge/
The penis basically consists of 3 spongy balloon-like chambers that fill with blood on erection.
See Diagram: http://en.academic.ru/dic.nsf/enwiki/265769o
This page shows the functions and mobility of the foreskin: http://www.circumstitions.com/completeman/
This shows how the foreskin is attached to the glans by the frenulum: http://en.wikipedia.org/wiki/Frenulum_of_prepuce_of_penis
Masturbating can be damaging to the penis if you apply too much pressure. It damages the spongy tissue chambers of the penis. Consequently the 3 spongy chambers do not fill out with blood and expand the skin. This leads to a tight foreskin.
A sensitive glans (head) is a sign of damage.
So what you need to do is allow repair to happen. Don’t masturbate as much. And when you do masturbate, don’t do it for a long time.
Too much masturbation (especially with the wrong technique) can also cause micro-tears in the skin, which form scar tissue; scar tissue is less elastic than unscarred tissue.
So, make sure you are not using a damaging masturbation technique. Just lightly grip the penis and move the foreskin up and down.
Once the inside structure starts to repair, the 3 spongy chambers of the penis will inflate to their proper position and this should expand the outside skin and there will be extra ‘give’ when trying to pull the foreskin down. In time, it will then be easier to retract the foreskin over the head.