Stuck in low gold

Ok...so I've been playing League for a while and I got stuck in Gold IV (right now demoted to Gold V) My rank extent was gold IV 75 lp but I lost it... so could someone watch some of my matches and tell me what I did wrong? I currently want to get to Platinum V .. I admit that most of the times it's me, not my teammates , atleast most of the times.. Could i get any suggestions? I play top,jungle and very rarely support.. I play Jax, Garen, Darius , Leona and Olaf (the meta guys) So the way I got gold was .. I played 10 placements.. got silver 3 bcs last season I was like silver 1 and I kept playing .. lost win lost win and I got to a 15 match win streak , getting in gold V with a 88% winrate and a mmr of gold 3 and ohh boy I started losing after that... SO PLEASE CAN I GET ANY TIPS ON HOW TO GET BETTER AT THE GAME ... TO RECAP I PLAY JAX MOSTLY AND IM STUCK Thank you for reading xD (please somebody tell me what I'm doing wrong in my matches)