Thanks EK, glad you're enjoying the thread! It's a lot of fun and I'm learning a lot too!
1) I always keep a mental count in my head of the bet size preflop, and then multiply it by players. Then i track each bet and add it as I go. It's a good way to stay engaged with the game (always present to be counting) and also ensures you're only remembering 1 number.
Note: I am rather gifted at basic math so it comes easily to me, if it doesn't to you then just practice your times tables a bunch and it will become more natural. I do this every hand.
2) Block out everything else, put all your attention on the game, keep yourself present, watch the action. If you can't recall what previous showdowns were or who has been winning/losing the last 5-10 hands you're not paying attention. All your profits in poker come from being in tune with what's going on.
Next up, don't try to get fancy, keep it simple and you'll usually get the money in the end.
Finally, be PATIENT, don't be in a rush to win, and don't go into a session expecting to win x$. This puts artificial pressure on yourself. Just play the game as it comes to you and be grateful that you get to play this wonderful game. That helps me keep in a positive state.
https://www.youtube.com/watch?v=KwmfN_ELsjA&list=PLBrNnyWekagMm7W5FhyJsu7EBZKq7s_m8&index=8&t=0s
3) Same as I mentioned above, practice times tables, kumon math, khan academy, any basic math method will give you all you need. No need for advanced combinatorics tbh.
Hope that helps and thanks for asking such a great set of questions!!!