tf2 is an old game with a tired and very developed "training" base. The resources to improve in the fastest of ways (overall gamesense and map/positioning) have been rehashed and tweaked to the small trends over the years that are eventually hit upon again. example: shotgun resurgence after a looong while of med speed/gb meta.
The only way to affirm and ensure you retained knowledge is to put them to use. Newbie mixes are still around and low level pug groups are much more accessible than i have ever remember them being thanks to pugs.tf and once you are friends with a few consistent puggers you have a very reliable supply of varying (albiet still new-friendly) pugs.
Your own drive to train and grind is what limits your resources to improve, just being consistent gets your foot in the door and discipline brings the rest through.
edit: sorry kinda buzzed and missed the point. finding a mentor in game is much alike in real life. They appreciate your thirst and drive to improve, you learn from their experience of seeing how the (similar) situation played out from their time. You're very unlikely to find a mentor/coach for hockey in a dive bar (pubs); they might frequent or pop in here and there but its not the best place to look for it. Out closer (pugs, these forums, people who do comp already) to the rink (already on a team/trying out, actually ingame) will always be your best bet.
the last time I seriously played or put time into reviewing was back in 2014/15 and while I can be very specific with how a certain hold or push will likely turn out; I cannot say as to individual responses from classes/positions. Mechanics have had their standard pushed so hard theres very little to introduce, but recognition is not the same as recalling and fully understanding the concepts behind them.