It really sounds like you're playing champs that don't fit your current play style and skill level. Try out something completely different, then later, come back to those champs you've been playing and you'll see a spike in talent. For myself, I started out trying to ADC and was getting blown up. My problem sounds a bit like yours, I liked an all-in style, but ADC shouldn't do that unless they have a flat out advantage in trading/cc. Same thing was going on with nukes. I swapped over to tanks and BOOM did I blow up the enemy team! It just fit with my play style so well, I was disruptive, I lived longer, I piled out damage. Later on, I started working on mages and found that I was quite good with several of them just from the sheer practice I'd put into LOL and the confidence boost of having had so much success on tanks (plus, having played against other champ types, I knew what they could and couldn't do, which benefited me tremendously.) After mages, I worked up ADC and did much better (still not the best jungle camp clearer, but I can gank hard core,) and, surprisingly, found that supports are possibly my best champs. Who knew? You sound like you're in the same boat I was in when I first started out, so I'd agree with the others that posted earlier, try out a tank (swing to bot lane until you're more practiced.) I'd suggest building health and armor/mr with a nice set of boots (counter the type of champ you're against) to get you started, then branch out a bit depending on the tank you're playing and the champs you're against.
Also, possibly the best advice I can give is to not limit yourself on what champs you play (to start.) Try out every champ a few times, at least. Not only will you be surprised which champs you find "easy," but you'll get invaluable knowledge of their strengths, weaknesses, skills, opportunity windows (both for and against them,) and build paths. I just can't stress this enough. Playing a champ lets you know how to play against them......after all, you will know what killed you easiest, right? :P
Basic tips: If you're playing an ADC, glass cannon sounds wonderful, and can be if you've got the high level skill to pull it off, but a mix of AD/Defense runes will serve you better as a start. Same with items; getting one defensive item can lengthen your time alive, which will increase the amount of damage you output over a glass cannon. This principle goes for mid/top lanes, too. Glass cannons are just that, glass. A burst or sustain type opponent will thank you for it and walk away with all the kills and CS they could hope for.
If you're having trouble with CS in any lane, try to make it a priority over harass (unless the other champ is very weak against harass) and don't be afraid to use your AA AND your abilities to last hit. Since you're not harassing constantly, you now have more mana to dish out in minion farming. It's more important to get those cannon minions and their lovely gold than it is to hit the enemy champ who is just going to chug a health pot anyways. You should probably still harass a little to keep the enemy honest and from getting a bully mentality, but just ease back on it a little until you've gotten better. Also, click on the minion you're looking to target so you can see his actual health numbers (later, you won't need to do this, you'll be able to tell just by the length of the health bar,) and watch the numbers above the minions you hit. This will let you know how much damage you're doing currently in the game to a minion with an AA or ability so you know when to last hit. And prioritize minions in this manner if you have to choose: cannon, melee, caster. They're worth the most gold in that order.
Watch your minimap. Take every opportunity to watch it and keep track of where the enemy is. Combine that with some defensive wards and you should stay much safer in lane against ganks. Don't be afraid to B. It's better to B and come back healthy than to try and overstay because you're afraid to lose out on those last two minions. More will come. And don't overextend. Pushing a lane into the enemy sounds like what you're supposed to do (the whole goal of the game is destroy the enemy's base, right?) But often, it's not. You get far away from your tower and have to run all the way back, being harassed the whole way by the enemy, often dying. If you've got good wards and know where the enemy is (especially that pesky jungler who loves to come and cut your legs out from under you) and you've got a good leg up on your lane opponent, go ahead and shove the wave to do some damage on their tower, but keep farming. Farming is going to help you more as a starting player.
For champs, I'd try taking tanky bot laners, like Leona (cast that shield, sword in for a stun, stun them again, walk away as the shield does more damage and all the while your ADC does their bit from the back lines,) safer mid lane mages if you're determined to go mid lane (for example, Ziggs can harass and farm fairly safely as he has a mine field to slow champs/farm, a satchel to either knock back the opponents or knock yourself back towards your tower - preferably both, and a bouncing bomb to poke or farm,) or a beefy or cc top laner (Mundo is fun and not that hard to learn, but Maokai is much safer as he can harass, farm, and root while also healing himself with his passive for sustain.)
Lastly, read a few guides. I know, it's boring and takes time, but some of them are really, really good and have great tips for those specific champs. Just make sure they're not old and out of date. And you can also watch pro matches to see how they farm, when they back off or initiate, what skills they use in what order, etc.
Best of luck, and if you need ingame help, you can always look me up if I'm online and I will play a bot game with you to help work out some of the kinks.
