It seems you haven`t let go so that is the problem, not an eating disorder. Sometimes feelings for someone will naturally fade with time, other times they take forever, so in the meantime try to find some interests that help you forget him. Perhaps therapy would help.
As devastating as it may feel right now, at the end of the day, he does not deserve you!
To leave you for a skinny pretty girl is very shallow and immature.
Why would you want to be with someone like that, right?
I really think that stress and your emotions are running the show, which is understandable.
You should't concern yourself with possibilities of having an eating disorder, as most likely it's not.
Start taking care of YOU, forget about your ex and move on to a better place in your life.
Therapy, like the previous poster mentioned, would likely help.
There's always a reason why things happen in our lives!
In this case it could be because he was not the right person to be with.
The opportunity presented to you through this "seemingly" devastating event in your life, is to help you make a fresh and better start and hopefully meet the person you deserve.
The MOST important thing you need to focus on is YOU, and making sure you take care of yourself and start eating several small meals a day. Rapid weight loss, not eating is NOT GOOD.
Healthy relationships aren't based on weight or looks. They are based on respect, kindness, mutual likes, shared values. ANYONE who bases someone's value in their weight or looks, doesn't respect the other person, and is NOT someone you want to be in a relationship in. If HE only is interested in how someone looks, he is NOT a good partner for anyone.
It might feel like she' won', but did she really?
I don't know how old you are, but please, don't look at this as a loss, look at it as showing you who he really is, and that you KNOW YOU deserve more than someone who doesn't value the relationship.