MMR Recovery

A few weeks ago, I queued with a friend of mine who is a division below me. Understandably, my LP gains became 15-16 after playing a few games with him. Keep in mind, I lost 3 games and won 11. After playing these games, I promoted to D5, so I could no longer play with him. After reaching D5, my LP gains became 15-16 at most and sometimes even down to 13. I then pushed through to D4 but still did not recover in MMR. Since then I have deranked because I lose 23-25 LP per game and gain around 13-14 LP. I end up going 6-3 one day and end up with only a few more LP then when I started. I am now hardstuck D5 with a 54% wr which is simply ridiculous. How can the matchmaking system think I belong in a lower division when I win most of my games? The games I do win I usually have a 4:00 or higher KDA.