OP - i agree with others, until you get your body fat down it's impossible to know if this is a head trip or something physical.
A couple questions to clarify your issue:
- Does your issue occur when masturbating or just with a girl?
- If this happens when you're alone, can you get a very hard erection but lose it in seconds without stimulation? Or can you only get a chubby type boner that's not very hard?
The first issue has to do with your penis's 'neck' muscles not trapping blood. This could be due to too much of a norepinephrine vasoconstricting effect. When this happens even a PDE5i won't work that well for you. This could be a hormone issue (see connection between venous leak and low T, low DHEA). The worst case scenario on this front is a venous leak issue caused by injury.
If it's the second issue then it seems like your body has weak nitric oxide production and a PDE5i will help. Also eating foods like watermelon (just buy bulk l-citrulline, it's cheap and easy), beets, etc, that boost NO production will help. But you need to lose the weight and get fit. A fit body with regular exercise is a NO producing machine. You could have artery plaque built up from being overweight that limits your NO production. You can reverse this with diet and exercise.
I would go to the site Peak Testosterone and read everything there. Then read this thread that is probably the best on the web I've found in explaining how erections actually work and different things to try:
http://www.allthingsmale.com/community/t...ons.19251/
Again, until you get your body weight down and lipid numbers in line - so you know your body can adequately produce NO - it's impossible to say for sure what's going on.