Getting LP back to where it was

In my experience it's how far over / under 50% winrate you are. Example when I was 49% winrate I would get 17-19. Once I hit 50% winrate I went back up to 20-21. Starting to go back below 50% again though >.>

It depends on your MMR: how much over/under it is compared to your current rank, and how much over/under your current MMR is compared to your opponents.

Your MMR is probably under your current rank. Keep winning to get it back up to your current rank. You can also play against higher ranked people to get your MMR up faster by duoing with someone who is a higher rank than you.

It is unlikely to ever go back to your higher level.

MMR gains are based on two things (a) your current leagues average MMR's distance to your personal MMR and (b) how many games you've played. (a) unless you suddenly get much more skilled or go on an insane winning spree this is unlikely to cause an increase since you are presumably close to the correct league. (b) will not increase till next season since the initial boost to MMR gains from post-placement will have expired.