Age 20 and Getting Erections To Often

Hi, im 20 years old and for as long as I can remember i have been getting erections to often and at random times eg. sitting on the bus and especially around girls which is really pissing me off. :$

What is wrong? What do I do?

Please help

Thanks

(PS. if its anything to do with masturbating, i masturbate once every 2- 3 days, is that the problem?)