This. I'm not dying as much as OP says he is, i did play a lot in the beta.In beta surprisingly people were keen on having at least 1 tank and 1 healer, even in unranked, i played probably over 200 games and amount of games that didnt have a tank or healer wasnt over 10, especially in ranked people were wary of composition, hero switches, situational counters etc. , of course this might not be the case with the amount of people and quality of people at release.
Dying too fast seems like an awareness issue to me, should be solved over time as one improves himself/herself, one of the most important things is to memorize maps and specific spots on those maps that are advantageous to each hero, like where is the point that widowmaker will sit on or where will she jump to when pressured at close range, or where are the spots for bastion/torbjorn that only has one angle of attack, from where can genji/reaper sneak to backlines so you can prepare in advance, these will become muscle memories and instinct over time as you play.
Recognizing movement patterns also play a huge role , like you see a glimpse of mcree runing past at distance from a window, there are two possible ways he can take , so you approach those point carefully as he will also try to track your movement and flashbang you from a corner as you come out, avoiding that flashbang is top priority