If she's 'fizzy' on just hay, it may be her personality/training rather than feed, altho it's definitely possible, as is a nutrient imbalance effecting her moods. Is she used to/comfortable with living in? Why do you have to bring her in at night? Is she/will she be getting much exercise?
Especially if she's overweight, I'd be feeding her just hay(small holed or double hay net), along with a good complete supp, which you could mix with a drop of oil into some alfalfa/lucerne chaff.
If you're wanting to put condition on, or give more energy & wanting to 'hard feed', grain is generally not the best option, as it is difficult for a horse's digestion, especially when fed in only a few meals or less a day, as a horse's digestion functions best on little & often.