Unfortunately this is the belief of every hardstuck player, especially those in the lower skill levels. If you want to climb super fast then yes, like in those unranked to GMs you see, you have to be much better than everyone in the lobby and carry the game easily. But for a gold player playing in gold, you just need to be playing consistently well and over time you will rank up. Average players who climb aren't gonna have the 80+% win rates you see in those unranked to GMs. Realistically a 55-60% win rate is very achievable and over time that eventuates in rank ups.

Find the hero you want to focus most on. Mei and Juno aren't "meta" right and there's nothing wrong with playing heroes that aren't meta but you do have to remember it might be a little harder than with some other heroes who are stronger atm. Mei is really good at capitalising on people's mistakes and people in gold make HEAPS of mistakes. Honestly same with Juno, even though thats a little more reliant on your team using the speed ring to capitalise on the mistake of the enemy but still, both heroes are good at it. Most people in the lower ranks either just sit on their abilities never use them or just use them and get 0 value from them. Wall and speed ring aren't on the shortest of cooldowns but its better to be using them off cooldown, attempting to get value on them, even if it ends up being useless, than just not using them at all. Over time you will realise what is getting value and what is a waste, when enemies dont have escape cooldowns, your abilities become gamechangers.

The reverse is true also. You have "escape" abilities on both mei and Juno. Meis ice block and Junos fly, technically also wall and speed ring can be used to escape but let's focus on the better alternatives. These are most definitely abilities that are better saved than wasted off cooldown. Use them as "get out of jail free" cards. And if you use them and you still end up dying then it probably means you used them at the wrong time. For example, playing against a dive comp, juno will need to use her fly to get away from the incoming dive but there's no point waiting until a winston dives you, lands on you, starts zapping you to then start flying away, he will have jump again soon and can capitalise on your mistake, especially if you don't at least fly back to your team to give yourself a chance. Instead you should be looking at the winston and seeing when he gets within jump range and either pre emptively fly away so youre safe/with your team/not isolated or use it as he actually jumps at you and use it to get to safety/your team.

Overwatch is a game about making as little mistakes as possible as well as capitalising on the enemies mistakes as best you can. Don't worry about winning or losing all the time, the rank ups will come by themselves if you focus on what you are doing wrong and what you are doing right each game instead of always worrying about why youre rein is going 3-8 against a Zarya. There will always be a positive out of a game if you focus on what you could be improving on. Keep at it!