I've been on dates with the women you describe Cam (maybe not overweight) and I know what you are talking about. Unhappy, self centered, depressed with the world. I was really surprised by amount of women I dated who showed these signs.
The best advise I can give you is keep dating, don't settle down for women like that. It's a good thing that you have a natural ability to recognize them, some people just marry them and then spend the rest of their lives in hell. There are women out there who are genuinely happy, optimistic and don't drag you down into a hole with them. They are rare, but keep looking and you will find them.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~