How to play ADC without support?

I need help. I played supp a lot in ranked. Like first 20 games but then I got a bit bored and played other roles too. And I have huge problems as adc. I'm way too support dependant. I'm fine when I have support who goes in, tanks and tries to save me when something goes wrong or keeps me alive (or at least tries). But I'm horrible when I play with supp who isn't supp in my eyes. Just my opinion, don't flame playing off meta etc. I'm facing eg Thresh. He has mobility, stays way forward, his adc is behind him. He zones me hard. He harasses me and I can't farm at all or hardly. My supp is full ap Vel, veigar or so. Stays well behind, does nothing to help me and/or goes away and take kills everywhere. When we are in team fight they somehow like forgot about me. Everyone runs around and then there I'm nuked by whole enemy team. This somethimes happens even with heavy tanks. We had Leona and we did well in bot lane but in midle of game she was just standing well behind of everyone and went in when enemies were all dead or almost dead. There was none to initiate. I was quite fed Vayne and their jungle Nunu and mid Cho were quite behind. So I went first, stunned Nunu, killed him, nuked Cho to like 10%, healed, flashed away and somehow survived. At this moment team went in and when they were almost dead Leona ulted. I just can't play when none initiates. Then we never fight and just wait for enemies to ambush and kill us one by one while we are falling back. I have bad experience when we are doing good and at some moment we just start to defend, go back more and more and they keep pushing us and we are like doing nothing. I do not mean fighting and rushing at all cost but I don't know how to play. I'm usually nuked as adc and I have no idea how to survive. I know I can't relly on team that they will help me, shield, heal or whatever. But I don't know why. I tried to stay really, really back but there is mostly someone just waiting for me, like Kha who appeares and kills me withing a second. I know it is long but I hope you understand what I don't know how to deal with.
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