Yeah, as an adc main, i struggled with this for a while. Playing melee range carries is hard for me, but I enjoy them and have mid with diana or fizz as my second role.
It's patience for me. With adc, I have a good understanding of were I need to position and when does that change (ie ultimates and cc down, i can be closer ect). This is the same for melee carries. If you go into a full team with their cc up as riven, of course they are going to cc and burst you down. You need to watch the fight develop, wait out the main cc abilities, then try and get a priority target. Bonus points if you flank the carries on their team. Basically, if you are used to adc like me, I think about what I really wouldn't want this fizz to do if I was adc on their team. I would want him to try land a max range fish i can dodge, and get noticed by my team when he tries to go in, so he gets cc and one shot or has to flash out. What i really don't want fizz to do, is wait out my supports CC, come from behind our team and one shot me before any members with CC can react. Maybe they are focusing something else entirely?
Just because you are melee does not mean you are front line. If your team gives you shit for 'doing nothing' when you are waiting for a good time to go in, just mute them they are stupid.
Think about if you have like a 'dive buddy'. Like a tank you can follow behind, or another assassin if there is no tank. Like, if i am playing a ranged carry mid or adc, i can maybe flash away from just riven. But if a zac jumps in at me, and I have to flash, then riven comes in right after, I am most likely fucked as the cc has just been used to get this zac off me. Likewise if it's 2 assassins trying to flank. If rengar ults onto me and I flash, my support trys to cc them or something, you come in and i'm a free kill, then the support is also unless he's really tanky.
So yeah, main tips are to watch the fight. If you playing locked screen then you need to try to not do that before teamfights. Its ok to space or y to lock screen when you are actually fighting. but before you go in you want to watch for abilities/sums used.
Flanking/not being front line
Diving back line with someone
try these, im not like some melee god, so i might be wrong but it did help me.
gl