A chunk of mastering World of Tanks is reflexes - how quickly and precisely you can aim, your reaction speed, ability to maneuver past obstacles, etc. You probably have no problems here.

Another chunk of mastering WoT is knowledge about the tanks themselves - the mechanics of penetration, armor angling, how various types of shells work in different situations, weakspots, getting hull-down, etc.

You probably have some of this mastered, but since your highest tier tank is tier 8, you probably have a ways to go.

A third chunk to WoT is seeing the battle as a whole - recognizing when it's safe to attack, when it's time to fall back, etc. Knowing in advance when your team is out of place, and what the likely response should be. Recognizing when you can safely take that aggressive scouting run, and when it will leave you a smoking ruin. Recognizing when to charge, and when to try to play defensively, stall for time, and so on.

Another, related chunk is being really good at the risk-reward tradeoff - "yes, I will do this risky thing, and it may not work, but if it does, it gives us a huge advantage".

And there's yet at least one other chunk out there that I simply don't know, which is why I'm a greenish-blue, instead of a blueish-purple.

What you're seeing, as you get into battles with tier 9 and 10 tanks, is that your ability at reflexes, which is critical at lower tiers, is not enough by itself at higher tiers. All the other factors rise in importance, because more of the people at higher tiers are competent with their reflexes, their knowledge of weakspots, map positions, etc. They are punishing you for mistakes you don't even know you're making.

Unfortunately, the best way to learn is to suffer through it, and try to learn from it what you can. If your team loses 5-15 or worse, there's probably nothing you could have done to win. If your team wins 15-5 or better, there's probably nothing you could do to cause a loss. But if you lose 10-15, and you're been dead since early in the game, that's probably something you did wrong that contributed heavily to the loss. Conversely, if you win 15-10, and you're the guy contributing at the end of the battle to the last few kills, you probably did something (or several somethings) right.

One thing I see from your stats is that you are playing a lot of different tanks. I did the same thing when I was working up the lines, and now I wish I hadn't, because spending all that time in mid-tier battles gave me bad habits that I'm still trying to break. And it delayed the "aha!" moments that I've only gotten by playing in tier 10 battles. The key thing about those "aha!" moments is that they also apply to lower tier battles.

Kewei, I believe, recommends that you simply focus on one line, and work it up to the tier 10, accept the suckage, and just keep solo-pubbing. Your win rate will suck, but you're going to be exposed to a lot more insight fighting at tier 10 than you will at tier 5 or 6. Over time, you should get better as you master some of the more sophisticated skills. And then, once you feel like you're finally playing reasonably well in that tank, you can start working other lines. And you'll start to do a lot better, because you'll have a lot more knowledge to bring with you, because you invested the time.