Some Advice Requested
Heyo,
I'm currently a Gold V, but I think that's more of a boosted elo considering I mained tanky supports to get up there.
Recently, I've been noticing a decline in my statistics and I've been losing more often, in either Ranked or Normals. I have been practicing with one champion in particular: 
...
...but I don't want her to be the main focus. I get most of her mechanics/techniques but I don't know how to use them well.
This applies to all lanes and roles: I don't really know what to do.
I'm a terrible midlaner because of my lack of experience and CS quality; a terrible jungler because I can't give a good gank, or when it succeeds I don't know what to do with it and keep it consistent; a mediocre ADC/Top but I stick to my lane to farm and lose all of it sticking with teammates; and a sideline support who does nothing but tank and/or heal for most of the game.
Whenever I am toplane or ADC, I can get kills. True, we have bad games, but I am confident that in laning phase I can score at least 1 kill, ahead or behind. My CS is on a completely different notion though. Every time we hit the middle phase of the game my income is based solely off of kills/assists. I can't seem to score any kind of CS without potentially ruining a teamfight that could have easily been won with 5 people. If I am splitting too much I get called out, but when I have to build something to counter the other team or to improve myself, I don't have any money. I try as hard as I can in order to reach the "100 CS per 10 minute" mark but I only seem to reach 200 at about 35 minutes on a lucky run. I've even tried launching custom games vs bots in order to practice but that never seems to carry over into an actual game.
I am always in bad positioning as a jungler. Either all the lanes are pushed up or they're ripe for a gank, and as I walk up to gank, I feel that I become sluggish and only manage to pop a low-CD ability. Whenever something good happens from that gank, I can't be consistent with it and I have to wait until midgame until nobody cares to CS, which refers back to the toplane argument. Also, almost every other game that I play with Vi I face against a Lee Sin. Apparently those two are destined to meet each other.
As a midlaner, uh...I own 5 out of 40 total (at this time). I don't venture much into the midlane considering the similarity of roaming and ganking with jungling and the fact that I can't CS as a mage, like with 
for example. Especially early. I dunno how people do it. For me, midlane was never something that I thoroughly enjoyed playing in, but I'd like to be a Jack of All Trades rather than a One Trick Pony (and the image earlier isn't really helping).
Supporting is boring. I'd usually take Sona or Leona botlane, some champions that I'm confident with, but that's all I ever do. Even so, I don't feel any sort of satisfaction helping my laner or my team. I feel like the expendable 5th person on the team who's just there to fill a spot. It seems more of a false victory when I win with supports.
My late game in general needs work too. I don't know whether to push to the enemy nexus, prevent a backdoor on our side, or when to secure objectives. I think I have decent map awareness, but that doesn't help me when I can see the Fiora top destroying the nexus and I can't deal any sort of significant damage to her to stop it.
Let's say for example that I'm Shen in this situation. I have 6/8/10 K/D/A and it's a pretty close game. My team is pushing to the Inhib turret while Fiora is taking ours, alone. I know that I can't do any damage to the 9/4/7 Fiora. We need somebody to peel our fairly squishy team but nobody else is paying attention. What should I do in this case?
All in all, I need help with improving my decision making, experience with roles, and source of income. Any advice?
TL;DR: I don't really know what to do with anything. Can I have some advice, even if it's obvious?