why do I lose 100-50 Elo when I lose and gain 50-20 elo when I win

https://support.chess.com/article/210-how-do-ratings-work-on-chess-com

How much your rating changes depends on the difference in player ratings, and their respective rating deviation (RD) values. The higher a player's RD value, the larger the changes after games. 

Players with very few games in a pool will have a high RD and high rating changes. As more games get played, the RD value will decrease, as will the magnitude of the changes. If you have large gaps in playing, the RD value will begin increasing again.

That's a main feature of the Glicko rating system and is designed to get a player close to an accurate rating within the pool quickly.

so how long until my elo gain will go back to like 9 or 7?

Just keep playing and it won't take very long.