Um sorry you are feeling that way. First let me say I think it us normal to doubt yourself with bug decissions like that. Second, don't let anyone beside you choose the outcome for your life. If you really don't think he is the right one for you, you can find another job, he may be hurt for a while, but will get over it. Also remember, if you get married now and decide he isn't right later. It will just be harder to leave, especially if you have kids.
My advice would be to talk to him, maybe give yourself some more time to decide for sure.
I felt some if those things with my dh before we got married. I told him how I felt and through that descussion I realized I really did love him.