I want to get my tubes tied, but doctor will not because of my age?

I'm 26, healthy physically. A few years ago I was pregnant but had a miscarriage. I want to get my tubes tied but my doctor won't because of my age.

I have good reasons. I do (did) want kids, but I feel very strongly about this. I come from a genetic nightmare. Alcoholism runs rampant on my father's side of the family. My mom was given up for adoption when she was born because her mom was a schizophrenic and bipolar. My mom is bipolar with a tendency for drug use and unsavory behavior. I'm bipolar, and though I manage it alright I do have a tendency to drink away problems (so I don't touch alcohol) and I'm self destructive. Add in some OCD and ADHD and you've got me. Sometimes I'm starting to worry I may have bigger problems, because I'm starting to show signs of schizophrenia myself.

This is a nightmare, even if I stay relatively normal - I'd be bringing a child into a horrible genetic disposition - and I made the decision that I wasn't going to. I'm in a long term relationship - I've been with my guy a little over 5 years - he has two grown children from a previous marriage and doesn't want more if I don't - and we getting married so it isn't a matter of my partner.

My doctor says I'm being irrational - but I wish more people would think about what they are bringing a child into before they have one. Crappy genetics, crappy world these days. Why add to it?

How can I convince my doctor that I've never been more rational about anything in my life?
*I have tried a few doctors. They will not because of my age / the fact that I do not have any children.