Use state machine for real states of your character like moving, falling, jumping, …
Use montages to react on something, like being hit etc.
And really important, use slots to define when a montage is usable while being in specific states.
Let’s say you are vaulting over a wall, you perhaps want to ignore being hit animations, so don’t use you “being hit” montage slot in the state vaulting state.
Furthermore start using custom blends between states.
And at last: Don’t expect that it is easy to do a complex animation system like in AAA games. It is much work , much tinkering and much to learn, like everything in game development.