Age slows everyone down but it sounds like you are only 10 years out of high school. That's not very old. I think that your lengthy absence is much more important.
Give yourself some time to recover your form and take advantage of things like Puzzle Rush to practice basic mates, forks, skewers and so on.
@1
"I forgot all those opening lines I studied them again" ++ Not useful
"I find it hard to remember even just one variation" ++ Chess is a skill game, no memory game
"I also find it hard to find checkmate in my game" ++ Think longer
"it's hard for me to focus on my game and my mind like to wander a lot"
++ That is mental discipline. You are the master of your mind.
"maybe I just haven't used to thinking fast." ++ Play 15|10 time control.
@1
"I forgot all those opening lines I studied them again" ++ Not useful
"I find it hard to remember even just one variation" ++ Chess is a skill game, no memory game
"I also find it hard to find checkmate in my game" ++ Think longer
"it's hard for me to focus on my game and my mind like to wander a lot"
++ That is mental discipline. You are the master of your mind.
"maybe I just haven't used to thinking fast." ++ Play 15|10 time control.
I know chess is not about memorization. But at least shouldn't you know some opening? Otherwise you're just moving your pieces to random square that may be good or bad who knows? So why is studying opening not useful?
So, back in middle school and high school I used to play chess against my friends regularly. And now, 10 years later since I regularly playing chess, I tried to get back at chess by playing online. If you look at my account it's quite old (2017) but back then I very rarely play online. Maybe once or twice every year or something. Only recently I started playing regularly again. And I noticed a different in my way of thinking.
Back then, it's easy for me to remember opening lines and recognize a checkmate pattern or finding a way to checkmate my opponent. But now, as I said, 10 years later, because I forgot all those opening lines I studied them again. And I find it hard to remember even just one variation. I also find it hard to find checkmate in my game. In a lot of my game where I win by checkmate, mostly it's actually accidental checkmate.
Other problem I have is it's hard for me to focus on my game and my mind like to wander a lot and that's make me lost a lot of time on thinking. Back then I never played a timed chess. So, maybe I just haven't used to thinking fast.