I can't cum without my own helping hand...help?

Okay, not even bothering to make a throwaway account for this.

For the year and a half that I've been sexually active (which may seem weird to point out but I'm turning 18 in literally a week), I've only been able to cum without jacking myself off to completion... once.

Some people have problems getting off to quickly? I have the problem of lasting too long.

Anyone dealt with the same thing? Any ideas?