I need some support advice

I've been trying my hand at ranked, and so far I find myself mixed and confused on what I'm suppose to do versus what I'm not suppose to do in the support role. I've been primarily playing sona and lux.

My last game, our jungler kept getting caught out roaming. Our bot lane would go to intercept, and my adc would be in peril too and start running. I'd go for the ult to secure the retreat, but die so two could live. I'd get yelled at for feeding.

Our last engage, it was me (sona), kog, graves, yorrick, and brand. Their shen taunted our graves who was out of position in the front. Their rammus taunted our kog. Their zoe had a sleep on our yorrick. It looked bad, and we were low. I flash ulted as sona. I cancelled a jhin ult, stopped the lux ult mid cast, and hit zoe. It stopped my team from melting, and 2-3 got away. I got raged at for inting. I see it as I saved my team from an ace.

This kind of stuff happens often. Where is the line between trying to keep your team "supported" and alive, versus just letting people die to their mistakes to make my KDA look good? Aside from the usual stuff like healing/buffing, and warding, trades and team fights seem to be the part where I'm either always struggling, or unsure what a team expects from me in my role. Either I don't get it, or I just have bad teams. Bronze/silver, just so that's out there. Any tips?