There are many good weight gain products on the market these days, some of which have already been mentioned.
My first thoughts are:
1. She is on a worming program and is up-to-date.
2. Her teeth have been examined and floated if necessary.
3. Lose the Sweet feed. IMO, sweet feed is the worst thing to come down the equine pike since Bot Flies and moldy hay
3.1 Again, there are a lot of great feeds on the market today. Look within a distance you can afford to drive for the best quality and price available for feeds that contain beet pulp or rice bran. Oftentimes these feeds come in pelleted form. They can be spruced up at feeding time with a warm splash of canola oil. I say canola because it is high in Omega-3 as opposed to corn oil that is high in Omega-6. While a good balance of both is necessary, if I am going to over-do one of them, it is the Omega-3
4. Might also want to consider a probiotic. It is possible she is a "quietly nervous" horse and something to help her digestive system can't hurt. 12 is not too young to be using some type of probiotic. There are many good ones on the market - I use ProDiZyme for my rescued Arab and my metabolic TWH.
Hope this helps you some