While I don't think this is the right section for this sorta thing.. and the forums aren't a blog as well..

Were you tanking? Healing? DPS? Assuming you were tanking from, well, tank section, and you've listed gunbreaker as your main class.
As far as performance.. Telling someone "go practice your rotation" is, in my opinion, a subtle way of saying your damage is low / bad. Now, if all the person(s) looking at numbers saw was your numbers, and didn't factor in things like your gear, comfort with your played class, comfort in that fight, etc, then frankly eff em.

Most ""mid-core"" groups don't seem to get what being "mid-core" means. As far as learning from the group, can you identify areas in which you feel you're lacking? Are you able to maximize uptime to the best of your abilities? Are you keeping your GCD spinning and not standing/moving whilst not using any GCDs? Are you stopping your GCDs while a mechanic is happening to try and see what's going on?

Nothing I see for you is open to the public so I can't just dig and find these answers myself, nor could anyone frankly, so again hearing just from you is only half the story. Especially this bit: "The reasons cited were that my damage was so bad that it seemed like I didn't know my class, and my inability to adjust on the fly if the boss was out of position or if a critical member was dead and I had to pick up something I would normally not have to. Additionally, I was told that I was not receptive to criticism of my play, and that it seemed like I was not putting in the same effort as the rest of the group."
My takeaways are:
>Assuming you were off tank; the main tank died to stupid and you werent as fast as they wanted in picking up aggro
>Damage can be factored in with lesser gear, but also were you breaking combos (i.e. breaking your 1-2-3 with a ranged GCD)? were you stopping your rotation at any point?
>Communication issues could be at play. They could have said something which to them was a point of "do this please," but that might not've been understood.
>How much research into the fights have you put, i.e. read guides on the fight(s) and the timeline of mechanics? Or were you just adapting to things as they happened?

Finally... its ultimately not that big of a deal if one static says their goodbyes. Just find another. But if the next one has similar feedback to you / your gameplay, maybe some introspection's called for. You say you're not *that* experienced with 14 playtime-wise, so having these moments of learning / growth are fine. Don't take it to heart such that you're disparaged from trying to find another group. Especially a ""midcore"" group. More often than not people aren't honest with themselves in what they really want. If they weren't happy with how you performed, take the time to see what useful insights could be gleaned from their feedback versus your gameplay, and look for a static that would be willing to help you improve.