Oh.. have i been down that road. (Many times)
For weight and muscle- grain wise: beet pulp is a life saver. Rice brand, it does its job. Alfalfa is another helpful one.
Supplement wise- try something from smartpak, i go there for a lot of my problem. Also try seaching youtube, some people on there dont know what they are doing so be careful.
Work wise-look at this artical one question is getting back to work.
Seana Adamson also has other articals that helped me a lot metally though out that rehabbing adventure. But, we had the same thing happen to are horses so that makes it different for everyone.
Good luck! I have a few more tip. Contact me if you want more information.
Hill work, lungeing long and low and fresh green grass.
Hay and good feeding! Paddy's Choise