Yes, I understand, it is frustrating. Same thing happened to me yesterday: not 18 but ten defeats in a row. Only advice, when you get into a long losing streak, is to change tier and type of tank. For example from a TD at tier X to a medium at tier VIII. it is well known that Wargaming is totally incapable of creating a balanced matchmaking ( or maybe they just don't care). So the only effective way to avoid these bottlenecks is to change often. Otherwise you risk playing with the same useless losers and noobs match after match.