Where am I supposed to practice if not Quick Play?

The past couple days I've been working on practicing Symmetra. I adored her in OW1 but her new playstyle since the rework has never really clicked with me.

I don't really have any DPS characters I play well, so I've been trying to practice Symm to just get a feel of how she works and try to really put in the effort to learn her new kit.

Admittedly, it hasn't been going great. I do struggle a lot. I die a lot. I don't always deal the most damage. I don't always get the most value. There's a lot to consider with her kit and it's more than I'm used to.

I've been getting flamed by teammates and even opponents. Today in a game the enemy tank kept saying how I may as well have been helping his team win, and my team was joining in saying how awful I had been playing.

It was Quick Play, though. Where else am I supposed to practice a hero if not in Quick Play? Am I doing something wrong by playing subpar in that mode?