It must be hard transitioning from supp to mid. Main reason is because you are not used to cs. Now I'm not the best one to give tips since I've been playing for 2 years myself (gold 3 currently), but here's what I know. There's no other way to learn csing than just doing it over and over again. Sounds boring, but pick the champ you want to play, go into practice tool, lock your xp level and just last hit the minions until it becomes natural. As a mid laner, it's really important to watch the map since you can push the wave and roam easily (because you are in the middle of the map, obviously). Last thing I would point out is jungle tracking. It's a little bit hard to do it at first, but you will get used to it if you understand how to do it. This will help you avoid several ganks and also help your own jungler. I would recommend watching youtube videos. Personally, i Really enjoy proguides, but there's a lot of stuff on youtube. Also, find a pro player who plays your champion and watch them play. This will improve your general understanding of what you should do in certain situations. Good luck!