Expanding Champion Pool in Snowball Meta?

Now, I'm a scub. Scrublord, even. Unranked, terrible at the game, absolutely bollocks, really. If I'm not doing stupid things and getting ahead because people don't know how to deal with it to get my wins, I'm usually losing. That said, I'd like to get better and one of the ways that I want to do that is to pick up a couple more champions to work with. Not that I don't understand the benefit of picking something up and learning it super well, mastering one champion's abilities is a good step in the way of improving, but there's something to be said for having more than one or two picks for any given role available. What am I supposed to do if it was already decided that I would go mid and someone banned out my Pantheon? No one bans Pantheon? They should, he's awesome. Not the point.

Maybe it's just at the low rank that I'm at but it seems like games don't last as long anymore. One way or another, there's usually a surrender or win within 20 minutes to 30 minutes in game time. Usually, whether my team is obscenely far ahead or somehow everyone on my team is doing awfully, including (probably exclusively) me. Not saying that throws and comebacks don't happen, but anymore it feels like there are fewer closer games and with the AI being as pants-on-head as they are, it's easy enough to go into a co-op game to try out a champion, get used to using their abilities and figure out what item build paths you like on them, but that will only take you so far.

So here's the problem with the faster games. Using a champion I'm not 100% familiar with, that I might have played a few bot games on and learned how their abilities worked enough to want to start running them in some normals to get a better feel. Obviously, being new, I'm going to make mistakes, but one or two mistakes compounds so quickly, and then once you're behind, you're behind for good, it seems like. There's less chances to grab for lost ground, it seems like, and each learning experience becomes a 5 minute test run into your first slip up, and then a resulting 15 minute compounding cycle of getting murdered and crying into your fuzziest pet or loved one.

Has anyone else had this problem, struggling to learn new champions in the current game state? Any advice on how to at least have more opportunities to learn from the inherent mistakes and resulting losses in games that become so one sided? I try not to sound salty, but if an assassin gets fed or roams for a kill at some point and gets out of control, team fights are less a matter of positioning and choosing plays than getting exploded and planting your face into your palms wondering aloud about how things have come to this, and I'm not certain how to combat it at this point.