Think of it like a game of chess. You're asking what you do when most of your pieces are gone, yet the enemy has all of theirs. Not all games can be won.
All you can do is try to either focus on helping your strongest team member, or attempt to stall the one on the enemy team. Get a sightstone if you can and get extra vision out, and hope your team knows how to play for time.
Other than that just keep practicing, and if this happens regularly focus on learning champions who are capable of successfully handling 1v2 or 1v3 fights. If you can't rely on your team to do well it's your job to do your absolute best and hold the enemy off while your team catches up.
Just please keep in mind some games simply cannot be won. A good skill to learn is to focus on what parts of a game can be learned from, and how to let go of the rest. Treat each game as exactly what it is, a single game. Don't focus on streaks, just focus on the game at hand. It's always good to learn from your past games, however when you're done learning just let them go.