Why am i losing MMR if I win one, lose one, then win one again?

I believe mmr change is dependent on the average mmr of the team you're playing against, and if you're queueing with anyone at the time. For example if you're in a game and your mmr is higher than your opponent's, riot considers it a game in your favor, and the resulting mmr gain will be low, while the mmr loss will be high.

League's ranked system is designed to make it near impossible to move up from whatever mmr you end up with. The easiest way to do this is design a system that makes you lose more than you gain the whole one step forward and two steps back mechanic.