Open Question: How can I end my life-long constipation?

I'm 18 now and for as long as I can remember, I've always had trouble with bowel movements. When I was about 7 my mom would give me an enema because I wouldn't go, but those would rarely help either. At 13, I had a real bad spell of it. We went to a doctor, he took an X-ray and prescribed some steroids and said if it didn't clear up in 2 weeks, come back. It went away, but ever since then when something happens that makes me nervous, I'll get real bad constipated for a week or two. Even when I'm not nervous, I only manage one good bowel movement a week, but my parents won't take me to the doctor. I'm worried it may be cancer, but it's been like this as long as I can remember... Please HELP! P.S. I'm male if that makes any difference