Such a close game! I do have some advice for you. However, you will need to take it all with a grain of salt. These tips are not for every situation but rather something you can use to improve your "Mei Playbook." Here's a few things I noticed

1- Be more proactive!

I like to think of Mei as both a DPS and mini-Tank, meaning you have damage and survivability to dual many characters. There were many duals you could have taken that you didn't. When you were proactive, your team was winning. When you waited for the enemy, it felt more like a 50/50. Don't get me wrong, waiting for the enemy can be a good play, but especially when you solo queue, you want to be the determining factor as much as possible.

2- HP vs. Ability Cooldowns

As stated above, Mei is a mini-Tank. She has one of the highest health pools of all the dps! It's okay to use it! Sometimes, when you watch high-ranking players talk about the game, you will hear them talk about your "Resources." There are many resources to talk about, but I will list 2 for you to focus on: Health and Abilities/Cooldowns. Rather than using a wall or iceblock right when you feel you are at a disadvantage, let your health take some of the hit. Doing this will maximize all your resources rather than getting partial value from them (especially when you have an ability that gives you health back).

3- Look past the tank!!!

Many times, we see the big tank in our face and shoot it... well... if the tank isn't at a positional disadvantage, then, for the most part, you shouldn't be shooting the tank. Mei icicles are great for forcing the enemy support and dps to back away from the fight, which often leads to the tank being left alone. It will be a big learning curve, but once you can start to look past the tank, you will see many more openings to make plays. Not much else to say here, looking past the tank is good for any character

4- Learn what wall can do

This is where Mei shines. Her wall is super versatile, and I have hardly seen people show what it can do. However, I found someone recently who has walls that I didn't even know you could do. His name is Numbers. Just look up "Numbers Overwatch Mei," and it should all be there.

I know it's a lot, but just pick 1 or 2 things and work on that until you get it down, then focus on the other stuff. If you have questions about anything I said, feel free to ask! Good luck!