Hey Lovido,
Some good advice above, but it seems most have skipped a simple idea.
You need to train and practice to extend the time you can play and play your 'A' game. It s like any sport. If you are not used to playing soccer for more than 45 minutes, a full game will tire you. If you never run more than a kilometer, five kilometers will be too much to handle.
Try to listen to some podcasts which reflect poker players life styles and adjustments needed. You will find many discuss how they had to make changes from playing cash, when switching to MTTs. Why? Because you can sit and play a few hands then leave if you want in cash game. But if a player enters a tournament and runs deep, that might entail many hours (if a serious tourney several days and 10 hour days at that).
So, I would begin by (when you have time) extending your sessions. If you normally play an hour or two, aim for three hours. I had a break not to long ago, where I decided I would play for eight hours a day. At first it was a strain then I adapted.
Now, I sometimes cannot believe when the first hour break happens- it literally feels like I have been playing for ten minutes! All you need to do at first is condition and train for longer sessions, and then you will find the mental game improves also, as you will not feel so stressed/tired.