If you want your players to regain stamina quickly you need to change their training to fitness training instead of balanced or sharpness based. Your players will annoyingly drop sharpness very very quickly but at least their stamina will be full for each match.
I like to keep things quite realistic so I only use a 25 man squad in career mode. If you have midweek matches and you don't set your training plan to fitness focused then all your players will be unuseable mid week and you'll have to switch to a band new starting 11 which is again not very realistic.
What I've done this year is just left all players on fitness based plans and let sharpness drop to 0% because I can still beat the computer when ever I want even if none of my players get a sharpness boost. Where as if my players all take 3 weeks to regain stamina that just makes me angry.
The ONLY good change this year is that you don't have to play the annoying training drills at the start of every single career mode. All the rest of the changes are terrible. It's a joke when in real life some players play every single game in the season. Weekend and midweek games consistently and play at a very high level all year round and don't look like they need to be put in a retirement home after playing 45 mins. The stamina for the players on Fifa is just terrible. Young players with terrible stamina, why? Players with 50 stamina that can't even make it to half time without needing to be subbed off. How is this realistic in any way? Lower league player given very low stamina rankings to help keep their ratings down but in real life a lot of lower league players will play more matches per season than Premiership clubs. The stamina for players needs a complete revamp. Career mode needs some serious work because nothing worth while has been added for years