I feel, rightly or wrongly, that your unusual masterbatory technique has damaged the sensitivity of your nerves at the end of your penis.
One of the best, if not THE BEST, masterbation techniques for a man is do use the hand around the penis up and down action. Another is to use just a couple or three fingers underneath the penis and the thumb onto (i find that leads to explosive cllmaxs!!)
I am a member of a forum on another website which focusses very much on penis develeopment and penis exercises and one of the best techniques for ensuring cllmax and maintaining erections is to deveop the PC muscle via PC exercises.
This is done, as I am sure you know, by tensing and releasing the PC muscle in 5 second intervals. Over time this will develop a strong muscle and increase erectile retention.
I must say I have a similar erection problem, being only able to maintain one during intercourse for betrween 20 and 30 minutes but I have found that the PC exercises are helping.
Avoid v1agra as someone rightly pointed out the side effects can be fatal. If you must try it then start with the 25mg dosage. I have some (my doctor - a lady doctor - gave me 100 mg tablets and I only use a quarter of a tablet at a time) and the really do work for me to keep me hard for over an hour at a time but my lady friend does not like it when I use it. She says it does not feel as nice as my normal penis -- far to hard like a painful rod.
Best of luck and keep me posted directly via email if you wish.