How do I get back into this game?

I officially quit after season 3. Mostly personal reasons, but I came back once or twice for a good game of nostalgia. So now I'm back and at it, trying to play seriously again. But its basically a whole new game.

How can I relearn everything when I do normal matches I'm being placed with diamon, plat, and a few gold players. Which is the level I felt I was at in season 3 but definitely I won't be up to snuff just yet. Is there an all reset? Do I make a dummy account and try not to be a Smurf? How can I ease back in?

I understand there is a big burden of knowledge with the game and ill learn most of it by playing. But I feel bad ruining 20 games to get back on track