I've been there. Here's how I got out:

1. Remember: the four idiots on your team with you are, on average, just as bad as the five idiots on the other team. The only difference is you.
2. Positivity! I'm serious. Don't. Fucking. Flame. No Dota player in the history of the game has ever played better after being flamed. If you flame, you are actively contributing to losing the game. Get on the mic and be a cheerful, supportive leader. Call plays. Congratulate your team. Keep the mood up. Yes, they're morons, yes, they're feeding, but the only way you can make them play better is through positive reinforcement.
3. Learn all the roles. If you slam your favorite role every game, you might be pushing somebody who can only play that role into a role they suck at. Every role is impactful, and learning them all will let you set your team up to succeed.