I completely understand your problem. I have the same problem to evaluate, when to join a fight, if i play squishys, with assassinstyle (diana, talon, nocturne, eve). I needed a long time to understand, not to pick these anymore, if i want to win. There are similar problems with annie or fiddle in teamfights. You have to decide, when to jump in with fiddle or when to do your combo with annie because both blow up fast if you made a mistake.
My recommendations are to stick to other tanky champs, which have staying power like Zac, Mundo, or even Jarvan, (I play jungle a lot) who can escape really easy if something happens. Leona and Alistar arent that good choices for this, but its a harder decision to go in, because they are only very tanky for a short amount of time and they dont have easy escapes. So you need a lot of farm or practice very hard to be successfull with them.
Other Champions, i had great success with, were long range poke champs like cait for adc or Xerath (perhaps wait for rework) and Zyra for mid. You can farm kinda safely with all of them and you dont have to decide to all in, because you can stay back really far and just hit the enemys. For Zyra i adopted a special build from a friend of mine, who played her alone for over 1000 games. I think liandrys and rylais would also be possible with the new support income.
Masteries 21-9-0. Runes: mpen/arpen reds - armor yellows - mres blues - Ap quints. For Zyra i always use the ultimate poke build with 2 Dorans Haunting guise into liandrys sorc boots and a Rylais. After that, void, rabadons and zhonyas in a reasonable order. If you get caught with Zyra in lategame, you can summon 2 plants and hit qer and all enemys near u will be half health and ready to get picked up by your team. This tactic won me a lot of games so far. At least you could try this in a normal game. Its hilarious, if one plant tragets the adc for half his health.