I don't understand!
My middlegame is worst part of my game. Does it make sense to exchange pieces/simplify?

simplify when you are winning
make complications when you are losing..
you will need pawns and pieces to create more complex positions, that's why you shouldn't do exchanges 'before' getting advantage.
usually +1 or +2 is enough to go and make exchanges, but until then, keep the tension.
@1
"My feeling is that less pieces means less variables" ++ Yes, simplification is often good.
"I would strive to win in the endgame." ++ Great
"Now I'm retired and I'm playing worse." ++ Two possible causes: decline with age and/or rusty.
Some players like Smyslov and Korchnoi played top level at high age. It is normal to think slower after a period of inactivity. So just play and it will get better.

Hello,
I created a Chessable course on this very theme of exchanging pieces: https://www.chessable.com/the-art-of-exchanging-pieces/course/82374/
Since piece trades connect to several key aspects of the game such as king safety, piece activity, weaknesses, etc. it is a great learning tool to study it, as we form connections and engage in deeper processing.

Before exchanging pieces, evaluate the position to determine whether it benefits you strategically. Consider factors such as pawn structure, king safety, piece activity and potential weaknesses in your opponent's position.

My feeling is that less pieces means less variables, less calculations, less pieces to monitor, less things to miss...What do you guys think?
I think this is a flawed mindset. I say this from experience because I too had this same mindset and it took me literally years of chess playing before I recognized it and did something about it. Usually, you want to trade pieces when up material (simply into a winning endgame) and avoid trading pieces when down material. There are exceptions to this though. For instance, if you are up material, but you are playing an attack on their exposed King, then this is one situation where keeping the Queens on the board might be stronger as this is potentially your strongest attacker. Just trading to simplify can slowly give away your advantage though.
This video of mine shares exactly this and the "Just Trade" chapter is precisely what this topic is about:
@10
"I think this is a flawed mindset."
++ I know international masters who play like that.
They chose openings like the Ruy Lopez Exchange 1 e4 e5 2 Nf3 Nc6 3 Bb5 a6 4 Bxc6,
the Petrov exchange 1 e4 e5 2 Nf3 Nf6 3 Nxe5 d6 4 Nf3 Nxe4 5 d3 Nf6 6 Qe2+ Qe7
the French Exchange: 1 e4 e6 2 d4 d5 3 exd5 exd5,
the Caro-Kann Exchange 1 e4 c6 2 d4 d5 3 exd5 cxd5,
The Sicilian Grand Prix: 1 e4 c5 2 f4 d5 3 d3 dxe4 4 dxe4 Qxd1+ 5 Kxd1
so as to avoid complications and go for an endgame.
"you want to trade pieces when up material (simply into a winning endgame) and avoid trading pieces when down material." ++ Yes, and you want to trade pawns when down material (simplify into a drawing endgame) and avoid trading pawns when up material.
"Just trading to simplify can slowly give away your advantage though."
++ If you give away your advantage, then it is not slowly, but by one mistake (?).
There are no bonus points for a faster win.
Checkmate in 20 moves or an endgame win in 60 moves, a win is a win.

I'm 73 next month so also not as good as I once was.
We'll show them who is the MASTER of senile dementia!
I turn 68 this year. Hit my rating peak around 62.

This is the phase when you should start thinking about CCAODW. Checks, captures, attacks, dealing with your opponent's threats, development, and waiting moves. In that order.

Hello,
I created a Chessable course on this very theme of exchanging pieces: https://www.chessable.com/the-art-of-exchanging-pieces/course/82374/
Since piece trades connect to several key aspects of the game such as king safety, piece activity, weaknesses, etc. it is a great learning tool to study it, as we form connections and engage in deeper processing.
I believe that we should avoid shamelessly advertising a paid course within these public forums.

Non mi capisci, non parlo inglese. Per favore mi parla nella mia lingua.
No me entiende, no hablo inglés. Por favor, hábleme en castellano.

My feeling is that less pieces means less variables, less calculations, less pieces to monitor, less things to miss. Before I would exchange, I would make sure that I would not be weakening my position, etc. My opening and endgame accuracy are ok. I would strive to win in the endgame. I have tried everything to improve, and I actually had a higher rating years ago when I was working and had much other stress in my life. Now I'm retired and I'm playing worse. What do you guys think?
Actually, endgames can often times be more complicated than middlegames; less pieces do not mean less complexity.
In general, if you are struggling with an aspect of your game, whether it is tactics, middlegames, endgames, openings, or something else, the solution is not to avoid it, but to confront it; to work on it. If, for example, you have difficulties playing sharp positions, the solution is to practice playing them.
I'm 73 next month so also not as good as I once was.
We'll show them who is the MASTER of senile dementia!
I turn 68 this year. Hit my rating peak around 62.
Korchnoi was a huge fan of porridge/oatmeal, said it built up his stamina for long games.
National championship win- at 80 yrs old.
I'm not sure I like chess enough to eat that stuff, even if I really liked Viktor the terrible.
My feeling is that less pieces means less variables, less calculations, less pieces to monitor, less things to miss. Before I would exchange, I would make sure that I would not be weakening my position, etc. My opening and endgame accuracy are ok. I would strive to win in the endgame. I have tried everything to improve, and I actually had a higher rating years ago when I was working and had much other stress in my life. Now I'm retired and I'm playing worse. What do you guys think?