Honesty is the best policy, so many relationships don't last because communication is gone.

If the sex is shit she has to know she needs to work on that and you're unsatisfied. If she's fat (and you're with her) she has to know she needs to lose weight if she wants to stay with you

Being honest about what you want and telling her she'll be honest with you. Then you can find out where she's at. Huge problem with relationships today is people just put up with each other and suffer in silence so they're doomed for failure, don't let that happen!!

Hope this helps