I've just recently gone through a little bit of weight loss with my mare. Before I got her, she was quite overweight (pasture ornament!). When we started working, she lost some weight & then it dropped quite dramatically. We had a little bit of a health issue in between too, which didn't help matters...
Like others have said, getting her teeth checked & a FEC should be where you start.
You said it's only been a month & a half---remember, it could take some time for your horse to respond & all horses are different. You might have to experiment a little to see what works. I'm convinced my horse, who used to be an easy keeper, just wasn't getting enough calories when she started working. Also, for good measure, I have her on Dr. Valentin'e EPSM diet (high fat, low starch/low carbs), which has helped with her weight as well!
I've had success with Empower Boost (gradually building up---we're at 2 lbs/day now) + "snacks" before & after riding of a few measured cups of alfalfa/timothy pellets with canola and or CocoSoya (horses love CocoSoya & it smells so good!) oil mixed in (I've got her up to 2 cups of oil per day & ).
I also treat her with Hi Fiber Gold chopped hay (Lucerne Farms) for snacks & even though she's on pasture, she's still getting hay until the grass really comes in. I've been happy with the way she's filling back out again. We have a little way to go still, but it's moving in the right direction!
We really had to experiment with her b/c she's prone to loose manure (which is another problem in itself!) if I add too much fat too quickly, but she's doing really well now. Beet pulp works for lots of horses...I didn't see a huge difference. The Boost & oil (& alfalfa pellets, I think) really seemed to help my mare. She also has a gorgeous shine to her coat b/c of the oil!
I think a faint outline of ribs is OK---but if you're like me, I don't like to see them...I certainly don't want my mare to get "fat", but I like a little bulk on her (she's a draft horse afterall!).