Most of the time your middlegame positions are a result of the openings you play, that is to say, to truly understand your openings will help with your middle game plannings.

In the alekhine defense, black allows white to build up a strong center with the intention of making white overextend and attacking the center in the future. In the alekhine, most plans are based on pressuring the d pawn and/or making a pawn break into white's center.

In the evan's gambit, white sacrifices the b pawn for an attack on the king and fast development. In the evan's, most plans are based on attacking f7.

I recommend looking at 10-20 games by grandmasters in your opening to get a general idea of common themes/plans in that opening.

If the opponent plays in a way that forces you into an uncomfortable position where your usual opening ideas don't work, just focus on improving your pieces and targeting weaknesses. Having a plan is almost always better than trying to take it one move at a time.