Motivation/Time per move

I have been trying lately to study the endgame some and have found that unlike other positions I am able to sit and think about the endgame because there are only a few pieces on the board and I can manage considering each option for quite a few moves in. This has however made me acutely aware of how little I can sit and think about opening and middlegame positions.

There are so many options it is absolutely overwhelming - and when I'm playing an opponent of higher rating than me, it feels like it doesn't matter how hard I try. I have played my Macbook on average setting (Which is a ridiculously high strength for average mind you) and been unable to achieve a win ever. I have played games for an hour taking back moves and lost, and I have played games for three minutes and drawn. So my question becomes, where do I get motivation to sit and think about the position when it all feels very vague? as in, with the endgame I can consider all options. With the middlegame the amount of available choices makes it such that I have to play for positional ideas that can ultimately lead me to be very weak against their ideas. If I achieve similar outcomes with or without actually calculating, clearly my calculating is not accurate. How should I approach this?

On the same line of questioning, when I do endgame exercises I can effectively consider most continuations to quite a few moves as I said. Why is it that most chess players sit and think for a few minutes every move when they have already calculated the continuation? If it is just to assure your calculations are correct I suppose that makes some sense, but if they play a move you considered in calculating shouldn't you already know your response and be able to immediately play it?

anyhow, the main thing here is how in the world do I calculate in the middlegame? Help?