Forgetting Problems

Hello. I have been playing serious chess for like 2 years now I suppose and there is this one problem that I encounter in like every chess game. Like, after calculating one line and moving to calculate the next I just completely forget what I figured out from the first line.

Suppose there is line A which I found and noted some key points of do's and don't. Then I went to line B, and it wasn't even a second (I am talking literally) that what I got from line A was completely discarded by my mind until I made the move. And this is for multiple times in every game.

I know I can do something like "note down what you saw" but I am not very confident if it would even work because I have tried stuff like that before. Even notes like "calculate properly" (this was when I used to end up missing a lot of moves) never worked because if I got even a bit engaged I always completely forgot everything other than the chess board and what is on it.

Could it be linked to focus issues? I do have them at a very strong intensity. I was prescribed meds for it recently which did make my concentration much better. I tried playing a game and did really well though it wasn't that this problem was occurring, I just was able to figure it all out before the disaster so it could also be that I just got lucky then.

If anyone has any tips on what I can do, please share it here. 

Before switching from Line A to Line B, say to yourself:

"Line A works only if he doesn’t have Ng4."
"Line A looks promising — favorable endgame."