So let me get this straight;

You are better than your rank, but can't actually make the climb to the 'supposed' rank you deserve? Sounds to me like a classic case of the Dunning-Kruger effect.

Most of my friends are platinum, and when we play together, though I'm almost 1000 SR lower, even when in competitive I can play very well against the enemy team.

You teammates are carrying you and peeling for your mistakes.

I personally would argue I should be gold or platinum (though I accept my rank) and here's why

Dunning-Kruger effect in action ...

That being said, I main Reinhardt and noticeably do way better with him than any other hero. However now I'm playing against diamond rank. At this level I could only play Reinhardt.¨

So you are a Diamond rank Reinhardt stuck in fucking silver? How does that even work? People have climbed to top 500 with Lucio and Mercy --- Why? because they are that good - you on the other hand aren't.

Listen -- You are not Diamond rank - You've had 3 seasons to prove that you belong there and so far based on your story you haven't made the climb.

So far the only time you do well, is when your teammates peel and play around you. (read: they are so much better they make up for your lack of skill)

You may not believe me, but the beauty of what i say is that you don't have to.

Because at the end of the day - you'll still be stuck in your rank hopelessly thinking that it's everyone else's fault than your own; while the rest of us focus on our own improvement and bear the responsibility ourselves.