MMR confusion

I have always had apparently low mmr, i just don't understand why. Last season before i quit i was d5 but was rewards +0 points per win. This season im in a similar situation where my mmr is low. Im currently plat 1 and i get demoted after 1 loss at 0 lp. im told this is because of my mmr. I heard that mmr also has to do with your win loss ratio and mine is 209 wins to 173 losses. i have been demoted from plat 1 several times now and it's becoming increasingly frustrating. can someone just tell me how i can improve my mmr. Winning the majority of my games doesn;t seem to help a lot.