My horse has had these problems too, it takes awhile and you just need to be patient.
1)Work with walking, trotting and cantering up hills, and walking slowly back down.
2) Ground poles help alot as well, start with one, and slowly work up to three or four, each 4 feet or so apart, is helps them learn to bring up there back legs.
3) Backing up, side passing, leg yields, bending helps loosen them up and makes them more willing.
4) Small jumps can help, if you know how to jump/training a horse to jump
Hope this helps--good luck, if you need any more help you can message me! :)