Do what I do when I go on tilt: think happy thoughts.
Your happy thoughts could be of anything, but I highly recommend thinking of tits. So every time you die or make a mistake just type out tits and you'll immediately calm down.
I'm unranked. I played bots for 2+ years. I am recently getting into PvP and am having great success at it. I had 2 years of bots to prepare so mechanically I am winning or holding my own in most of the matchups I find myself in. That being said I am also learning Zed. He is very hard to truly master and takes a lot of practice to get a feel for timing his shadow swaps and when and how to use said shadows. I found a small Zed series that helped out for teaching basic Zed combos that really improved my game practically over night. Here's a link to the first video:
https://www.youtube.com/watch?v=xtCHRqr-jLU .
They're super short (about 5 minutes each) and pretty basic. In the Bronze ELO sometimes the basics can go a long way.
As far as last hitting goes Zed actually has a pretty easy time last hitting via his passive. And his shadows can be used to farm the waves as well. But one very useful tip I can tell you about is how to farm under tower. When you have a full hp minion wave at your tower what you want to do is use this strategy in order to ensure you get the last hit. Melee minions are safer to farm. Let the tower shoot the minion 2x then auto attack after the 2nd shot. You should now have that CS. Caster minions require a bit more effort. Attack it once when it has full hp. Let the tower shoot it once. Now you auto it for the kill gold. So melee minions 2x turret shot, then AA. Casters AA, turret shot, AA again. The cannons have enough hp that there is no failproof way on how to farm them properly. Every champion has differing auto attack damage rates, there's also minion aggro involved and you just gotta watch them in order to take them out.
As far as simple mid lane champions go I learned the role on Lux and Morgana. I bought Morgana first, I heard she was "The poor man's Lux" and a lot of the skills I learned on Morgana translated over to Lux. Then I bought Pantheon and Kha'zix as my "AD mid laners" and nowadays I don't ever play them there, but that was my thinking back then. They still work in mid lane, its just awkward. After that I think I bought Swain and Karma and after that I forget who I bought. If anyone tells you not to buy a champ because they are weak you can ignore them. I've been here almost three years and every champion has had a point and time in their tenure when they were OP. Like Garen for instance. 10 patches ago he was garbage, now he is pick or ban. So when picking out new champions look at what the champions do on the spotlight overview, watch a couple youtube clips if the overview piqued your interest and then buy them and practice them. The only time you shouldn't buy a champion is if their price is about to be reduced next champion release (Lulu is the next champion that will get reduced) or if there is a rework on the way for that champion (Taric is due for one soon). That and NEVER BUY A CHAMPION ON A MONDAY. The reason being is that the free week of champions rotates every Tuesday. It feels really crappy when you buy a new champ on a Monday, find out they are free for the next week and you end up not even liking the feel of that champion. That's a crappy feeling.
Sorry if I got off topic a lot. Just trying to give you some stuff I picked up along the way.