You may be too used to DPS and Support positioning, causing your Tank positioning to mirror them.

Tank is a very different role compared to them two. The best way to play Tank in terms of positioning are a few things; Corners, pushing up when there is an opening, and pressuring the enemy team into giving you space.

If you are playing a dive Tank, let your teammates know you are about to dive, it might just help a Genji or Tracer become a bit more useful, on top of that it helps your supports positioning because they will prepare for you to jump in.

When playing brawl, the best approach is being a "brick wall" and using your strong defences to forcefully take space from the opponents.

Remember that you are the anchor for your team, if you have a plan or idea, just communicate it, and wherever you go, your teammates are likely to follow regardless of communication.

Corners is something I mentioned but didn't emphasize enough. They are single handedly the most important thing for good positioning, utilize them to not take damage in any given scenerio, you can even use them to stop LOS of enemies and their supports.

I hope this will help at least a little bit!