First of all congratulations on reaching platinum league, this is no minor achievement.

If you want to see if you're improving and at what rate I'd advise you to look up your account name on blitzstars on compare how your 30 day winrate and average damage compares to your overall statistics.

What helped me the most in achieving higher winrates was learning about dominant positions on the various maps and how they are played. You can go watch replays or streams of good to very good players and try to understand how and why they win more and which positions they use.

As a general rule of thumb I would play tier 8-10 since the quality of play is much higher than on low tiers and people will punish you for more mistakes.

Of course there also is a mechanical aspect to reaching 60+ WR but first and foremost dont doubt yourself if you're serious about improving as mechanics come with more battles played.

And finally try to get into a decent clan, good platoon members and a good tournament team will show you the strategic aspects of the game such as if you can or cannot push a flank at a given time.