How to raise my mmr?

Ever since I got to platinum, I've been getting 16-18 lp a win and -18-20 a loss. My last 3 games I got 16 lp from both wins and then lost 20lp when I lost one game. This makes it significantly harder for me to climb, so I asked some friends about this and they told me that it is because my mmr is too low. I read in another thread that winning increases your mmr but ever since I got into gold 2 to now I have been losing quite a lot(I am down to 60% win rate, had almost 80% before entering gold 2) and it's mostly because of shitty teammates. I have been improving myself as well but I can't win as much as I used to win so I think I will be stuck in plat 4 until the end of this season if my mmr doesn't improve. Check out my account if you don't believe me.