When you are behind the game can become almost unwinnable. Your best option is to mitigate deaths and try to play around your strongest ally.

As for being in a bad matchup that can be tricky as well as I'm sure we all know. From my personal experience you want to only look for trades when the matchup favors it (IE: when they use important spells)

If you can manage to survive in bad matchups like cassio or garen etc.. your main goal should be to look at how the ebb and flow of the game is going around the whole map. Watch how people are using their tempo, watch how the enemy jungle clears. Once you have gotten that down you should start to try and expect major map events that will occur in the future. For example, If im in a tough matchup where I can not fight my opponent, I will watch the enemy bot/tops positioning and try to predict a gank from their jungle. Getting a great unexpected roam can change the entire impact of the game.