You are expected to try your best in any and all games you que up for, even if it contains trolls, toxic players, inexperienced players or otherwise. There is nothing you can do without risk of Leaver Buster punishment to not "waste" time.
As for your mindset. You should hopefully be taking every game as a learning experience. especially the bad games. You learn more from Failure then you do from Success IF you are willing to look at it from an objective standpoint and not an emotional one. If you take the mindset of playing to learn, then there really is no such thing as "wasted" energy in most games if you are proactively looking for ways to learn and improve.
Playing a Solo lane also helps with this as you will then always in most cases have a laning phase where you can learn the match ups, where the trolls impact wont reach in most cases unless they are INTing and running down your lane.
Lastly if you have little time to play. I would suggest One tricking a champ if you can handle some degree of repetitiveness. If you can find a champ that you really enjoy a lot, and to the point you want to play the champ in to all situations, that can also help to make the game a little more enjoyable. As you might be able to guess from my name, I main Tryndamere with Trundle as my backup. I enjoy those two champs a lot and will play them into any situation and lane. The fact I enjoy the champs to the degree I do has helped me just ignore the trolls and focus on trying to do what i can do and what i can control. Maybe this might also help you.
