Help Mid life crisis at such a young age !?

I am a 25 year old female, i have worked full time since grad. from high school and i go to school part time. ( i have always been a worker bee , as my dad would say) and i am engaged to an amazing guy and we are just at the start of planning a wedding. right now we are just going over the must haves'. the last few weeks i go to bed crying. i keep having thoughts that i should just leave my job sell my car and travel. or i thought about going to school full time. i have never lived in a crappy run down 1 bedroom apartment and a part of me wants to. i have been with my future husband since i was 19. and i keep asking my self is the gr greener on the other side? my future husband keeps telling me i need to stop acting like a brat and except life. i'm happy with life but i keep saying what if. however i know i could spend my life saying that.. my fiance is 10 years older so he has 'lived life' ...any advice?