My top champions:
Riven
Nidalee
Malphite
Rumble
Irelia

Support:
Sona
Nunu
Leona
Janna
Soraka

It's not just mid I'm having these problems with. I need to narrow it down to just a handful of champions.

How do you pick your main roles and champions? I've done decently in every role with many champions, and it's so difficult to choose which roles to focus on, let alone 2-3 champions for said roles.

I also have a lot of difficulty deciding when to choose certain champions. Great, I got my role, but which champion fits it better for this team comp and against the enemy team?

Example: The mid champions I currently play are Diana, Zyra, Ahri, Katarina, and Galio. When do I choose one over the other? Normally, if I'm not sure how to play against the opponent, I pick Galio. If I'm first pick, I play Ahri. When do I know it's safe to play Diana, Katarina, or Zyra? Which ones do I drop?

Edit: Could you at least tell me why you're downvoting me? I really like a lot of champions. Why is that bad?

I choose my champions based on themes, skins, or lore. Zyra, Ahri, and Diana are all big examples of this. I loved the sneak peak for Zyra, and waited for it biting my nails for a long time. I'm really into plants, dryads (Dryad Soraka was the second skin I ever got,) and things like that, so I was so stoked for her. I was there the day Ahri was released, running around the forums like a chicken with my head cut off until I could buy her Dynasty bundle and play her. (First bundle ever.) I'm a big Leona fan, so when I heard about Diana I was so giddy! I went and made a PBE account just to play her (Dianas Panties.)

Anyways...yeah. That's how I choose my champions.

I have Galio because of his Hextech skin. I've only lost a total of 4 games with him. I got Anivia because of her Hextech skin. Same with Janna. (I'm a big steampunk fan.) I have a Nidalee smurf because I adore her, I played Katarina long before her rework...etc.

Elementz tier list.

Voted for everything as well

1. It's largely based on opinion
2. Most of the champions I play are already tier 1 anyways. :/

lol. Thanks.

Well, it's not too hard to pick a main role. First, you shouldn't pick "one" main role. "Main" at least two/three. Preferably one of the Solo Lanes, Support/Jungle, and whatever other third role you like. That way, when you go into Ranked, you can say, "Hey I'd like this role" and if you get it, Hooray, and if you don't, you can ask for your fallback role. And if worst comes to worst, just say you'll Support or Jungle. Most people will always let you Support or Jungle. That's why you pick one as backup.

Diana can beat most mids. The one's she has trouble with is mages who build somewhat tanky and have a lot of sustain. Morde is the absolute worst to play against.

Zyra can hold her own no matter what. She's similar to Morgana. If you can't beat your opponent, at the very least you should be able to sit and farm your lane all day until late game where your damage and utility rule the game.

You just need to play a lot of champions and learn their weaknesses/strengths. When I first pick of a champion I usually go through phases: Figure out how to play the champion (lol). Figure out the nuances to the champion. Play against various enemies and theorycraft based on your knowledge of the champion who you are good/bad against.

Galio is pretty solid but excels vs dot champs.
Zyra is good when they don't have a great ganker in jungle.
Ahri is a pretty solid pick overall. Just make sure you use her mobility to full use (gank other lanes).
Kat excels when their mid doesnt have an instant escape or cc.
Diana for squishy opponents.

You know that thing called enjoyment that you supposedly play this game for? You'll get a lot more of it while playing a certain champion, and then you'll play that champion more, and then you'll call it your main

Of course. Mid was just an example as it's one of those roles I have a million different champions for. Right now I play all of them, but jungle is my weakest. The only jungler I do decently with anymore is Shyvana, so I'll usually ask for "Anything but jungle." That's the advice I give my friends, too, but I have trouble following it because I honestly like every role. :/