Many times, the reasons people leave could be considered invalid. However, if they're not willing to work on the marriage (both parties need to work, not just one) then there is no use debating whether it's a valid reason. It's just their "reason."
If you're unhappy about the way you look, then you need to do something about it for YOU. not for someone else. If someone is only going to love you for the way you look, then they're shallow, and we as humans deserve more than that. If you're unhappy with your weight, why don't you join weight watchers or something? They have support groups which would be something very beneficial to you at this time.
I'm really sorry you're in this situation. Do you think you complain a lot? Are you complaining just to complain, or are there valid reasons? My husband told me I was miserable and complained alot too, but I feel I had valid reasons. That's a whole nother story. All I can say is I'm happier without him now. Life is much more peaceful.