It sounds like you may have a dissorder know as Peyronie's Disease you can get some free information about it and how to solve the problem here http://www.tinyurl.com/boston-medical this is the best information out there about this and other reasons you are having this problem.
How long has this been going on for? Do you not get erect at all, even when you are aroused? What about random erections or morning erections? Best thing would be to go see a doctor, and get a referral to a urologist who specializes in male sexual problems.