The Force seems to be a use it or lose it type of skill. The more you use it the easier it gets, while if you don't use it regularly, you forget how, or it takes a lot of effort to use. See how hard it was for Obi-wan to use the Force after about a decade of suppressing it in order to hide from the Empire. Look how tired Grogu gets every time he uses the Force up until after he has gotten some more training with Luke. After that refresher course, Grogu seems to be able to use the Force a lot more regularly and without getting tired. I wonder if Luke hadn't cut himself of from the Force for years after Kylo Ren was made if he would have survived his Force Projection in The Last Jedi.
Ahsoka seems to have been keeping busy since she became Fulcrum, as she has little to no issue with her Force powers. What she was doing during the OT is still a mystery, though I wonder if she managed to make it back to Lothal shortly after the Battle of Lothal and spent her time trying to train Sabine, only for the Purge of Mandalore to end their training. Ahsoka walked away, but then did what? Obviously she was no longer on Vader's radar as he was busy hunting Luke. And also obviously, she wasn't training Luke, nor did Obi-wan, nor Yoda, consider that a possibility. Possibly because of her issues with Ankain being Vader closed her off for a bit emotionally. She could still have been running the Empire around as Fulcrum. Keeping whatever the Empire was getting ready away from Lothal, allowing the Rebellion to use that planet for stuff. Likely building new starfighters. Maybe even the Alliances' training academy. Be a good source for new pilots for General Syndulla until they can get their hands on some giant capital ship they could use as a mobile academy.