There are many books written on this subject; Ron Klinger's 'Improve Your Bridge Memory' was the one I used, but I imagine your public library or bridge club will have a selection. The problem is that nobody but yourself knows what methods work for you.
Two examples from Klinger, one easy and one harder; you don't need to count how many of your own trumps (or other important suit) have gone, so long as you pause at the beginning of the hand, say to yourself 'I have four trumps and dummy has three; that means they have six between them', and then count each time one of the six is played. And secondly, always try to make up a mnemonic for a principle: playing Roman Blackwood, 5H shows two aces the same colour, 5S either two major-suit or two minor-suit aces, and 5NT two either hearts and clubs or spades and diamonds. That's hard to remember, but if you think of clubs and hearts as both a rounded shape, and remember C for colour, RA for rank and SH for shape, C-RA-SH should be enough to remind you what your response should be. (You make up your own mnemonics based on what helps you to remember, of course).
Personally, I found the second tip useful and the first counter-productive; but only you can tell what will work for you, and only after hard work both in finding methods and in practising. (Don't believe anyone who says 'it just comes naturally'; usually they have been playing for thirty years, and have forgotten how long it took before it all came together.)
Edit: I gather the real problem is remembering everything at once. In that case, try to build up your memory gradually, and try to separate the problems. So don't try to remember the exact probability of every possible distribution, start with '3-1 is 50%, while 3-2 is 60%'; you can add more, and refine the numbers, as you get better. And having made your plan based on the assumption that trumps will split 2-2, you don't need to remember why you came to that conclusion, just to count how many have shown up.
