Start with just upping her grain. Yearlings often get that belly when they are not given enough grain. Half a flake of good quality hay is good. I would only allow her on the grass a couple hours a day if you dont want her to have a big belly. You want soft hay, nothing real stemmy or weedy. Coarse stemmy hay seems to cause a pot belly too. Grow colt wont make a difference condition wise. Calf manna is nice (I feed it) but it doesnt take the place of feeding the right amount of grain. I feed it in addition to grain, its a nice finishing touch on weanlings and yearlings. I would up her grain, she needs it twice daily and much more of it.
My starting point would be not leaving her on grass so long, I would split her feeding of hay between morning and night (1/4-1/2 a flake am, and 1/4-1/2 flake pm) and build her up to the 4 cups of grain. A quart of trotter weighs 1.45lbs according to blue seal.
Blue seals feeding reccomendations for trotter are 0.70lb - 1.00lb per 100lbs of horses body weight. I would bet your girl is around 200lbs. For 200lbs of body weight for a yearling blue seal reccomends feeding 1.4lbs - 2lbs of trotter daily. The handful your feeding her is probably once a couple ounces.
If you are stuck with trotter it will probably be alright if you up her feed to the right amount. 1 handful a day isnt enough to make any difference, honestly, at that amount you might as well not be feeding it at all. I can about promise you that if you cut her back on the grass and up her grain to the reccomended amount for her age and weight that she will fill out, grow more evenly, develope more muscle and that big belly will go down. You might have to decrease or increase the grain a little after 30 days, but 4 cups twice a day is a pretty good starting point. I always take pictures of the horse from all angles the day I start a new feed, and then again in 30 days. Its suprising how much they change and we dont realize when we look at them everyday.