how can I find my niche in life?

I feel like I am mentally unstable. I know that this is part of growing up and finding yourself as an adult but I feel like I can not find my niche in life. I am grateful and blessed to have a job but it is part time and I make very little money. I am a college graduate with two degrees, a teacher's certification, and grad school courses. I am intelligent and did well in school, have a lot of experience yet I can't seem to be happy with being employed. Every job I get I find fault in it and begin to burn out after a month or two. This would be fine but I am a mother and I have responsibilities. My ultimate goal is to be a teacher and every time I feel like I am inching towards reaching that goal, something stalls. I have done everything I am supposed to do but I still feel like I can't get ahead. I am not fulfilled in my present place of employment. I am overworked, underpaid, under appreciated, and over qualified. I feel so confused because I feel like a failure. I am disappointed in myself. I get physically ill when I know I have to go to work. I sat in the parking lot at a restaurant on my lunch break yesterday contemplating not going to back to work. The money will come in handy for now but I need a change. Could there be something mentally wrong with me where I can't seem to want to work? I am an independent and driven person so why can't I seem to find the strength to want to work?