How can I fix or reset my ranking in Rocket League?

During the first set of games your rank is considered to be provisional, and is more susceptible to swings. Once you finish 10 and are ranked, it is still susceptible to more movement than normal. Once you finish 20 games, then it will become normalized.

That is the way that the base ranking system works that is used to calculate rating in Rocket League.

The rating system in Rocket League is a custom implementation of the Elo rating system.

Playing with a player of a different rating will effect the way that the shared rating points are awarded or removed. In the case you outline, yours either declined more because you had not reached 20 games, or because your rating was higher than your teammates.

Ratings do not get stuck though. The only place it is possible to not gain many points per win is at the top, where players can only get a few points per win due to rating difference (such as when the pros play); and similarly it is possible to lose points at the bottom for the same reason (that the rating difference is so large that you cannot get away from it).

Keep playing, perhaps solo queue in order to get a more fair representation of ratings on the teams. Your rating will eventually accurately represent your ability.