When DD was younger she had the same issues of throwing high one pitch and low the next. The way we worked on it was I would have her stand about 30' away and throw 3 pitches to me. (she was in 10u pitching from 35' at the time) Next I would have her move back to 40' and throw 3 pitches. Then 30' for three more moving her back and forth. Then have her throw from 35 a couple of times. When she was closer it just seemed easyer for her to hit her target. When she was farther then she had to throw hard to cover the extra distance. Along the way the proper release just showed cause she wasn't thinking about where to release it just throw. The other thing I would do is have her throw inside and outside while she was doing this. This is something we still do sometimes. I just make sure she is working on her other pitches at the same time, CU, drop... the only thing now is I wont let her throw from 30' I'm too old for that.