Thanks for your feedback. You are right about my playstyle, but i still want to play assassins and ap casters. Heres a few champs I'm looking to improve if you could help me by suggesting the proper playstyle and mindset for each: Zed, lee sin, fizz, leblanc, orianna, syndra, talon, kha zix, yasuo, and ahri.
The difference in playing tank initiators with high burst and assassins is mostly timing. With a tank, you're going in when you see a vulnerable target when your team is close enough to capitalize on it, and hopefully either killing someone or knocking them so low that they're now out of the fight, while you absorb most of the enemy team's CC abilities in their attempts to save whoever you went on. As an assassin, it's pretty crucial that you not go in when you can be locked up and killed. So what you need to do is pay attention to abilities that are being used, and everyone's positioning - if they blow their stuns on someone else, that's your cue. If a carry gets too far away from people who can stun you when you go on him and you can 100 to 0 him with a full combo, go for it. If their support is stunlocked by your frontline and won't be able to stop you from comboing a carry, also great. You should also try to pay attention to summoner spells if you can, specifically things like whether their carries have flash up, and weigh that accordingly - you don't want to go in if they'll just blow flash while you have a pretty high probability of dying.
The need to avoid CC as an assassin is also why flanking attacks are so strong - if you can come at a carry from an angle in which you don't have to pass through anyone who can CC you, you have a much better chance of instantly killing him then getting out before they can respond. Trying to flank splits you up from your team so this can put your team at the risk of them being initiated on while the entire enemy team moves away from you, and you're stuck in the situation of trying to run up to them while your team dies, but it's still a tactic you should try to master.