Learning Mid

I'm a support main and I played support basically from the moment I started playing, however I want to learn mid lane and I want to main syndra. My mains bot lane are Sona, Nami, and Morgana and I'm aggressive bot lane, but when I transition to mid lane I'm really passive all of the sudden and I don't know why. I know most suggest to play Annie, or someone easy mid lane when starting out, but I've been at mid lane for about 4-5 months and I'm still not getting the hang of mid lane and I can already play annie, and Malzahar. I want to do a champion who's a bit more difficult and does good damage. So how does one transition well from support to mid because there's lot's of differences between the two roles and, anyone have any tips for mid lane and learning/maining syndra?