Why can't I just leave

My fiance and I have a toxic relationship. He has some major issues that affect me so much. It's causes me alot of stress and anxiety and I just can't deal with anymore. He does things that aren't right and scares me sometimes. I always threaten to leave but he always manipulates me into not doing it. I just left my job to stay at home with my daughter so now I'm even more stuck because now I depend on him for everything! I really don't have anyone I can talk to about it. I want my life back with just me and my daughter but I'm having a hard time letting go. Why can't I just do this!!