I'm thinking that my fiancee is bad for my life but I'll never break up with her.
Why don't you want to break up with her even though you think she is bad for you?
I am not really sure why.
I am not sure why you feel you should stay with her, but you really shouldn't stay in the relationship if you think it is an unhealthy one. Marriage lasts a very long time and you may regret it later if you don't break it off now.
I don't care what you say. I'm going to marry her.
Marrying her would be a very bad idea if she is bad for your life. It will make things down the road harder if you stay in this relationship.