"men do not find fat women attractive" - that is not true.
I prefer fat women and don't find anorexic women erotic at all, it's just a preference. When my girlfriends gained weight I was always happy and told them that I find them more attractive with the new curves.
My last girlfriend made also a shocking experience like you, but it was a bit different: She opened a tab with fat girls.

Ask her why you need to fatten up. I would also ask her what happened that she has changed from a very fit boy type girl into a voluptuous heavy woman.

Don't worry, Bec.
You will put on the weight a lot easier than you lost it. When you have been allready fat you've got fat cells which are waiting to be filled up. If someone loses fat the fat cells are just shrinking and when one is getting fat again they are getting bigger and when they reach a certain size they are getting more to be able to store all the new fat. So I'm sure that you will get easily fat again and probably even fater than you've been ever before.
How much do you want to gain?