Can't get used to the game anymore.

So during summer a friend of mine finally got her first pc. it was a typical 200 euro pc with 4 gigs of ram and intel hd graphics. One of the only games we could play together was league. She already knew about the game so it wasn't that hard to get her into it. Near the end of summer or to be exact 20th of august my friend got herself new pc parts and she built a custom gaming rig yey. We could finally play other games aswell. We started playing a game called APB : reloaded (we both had played it b4 but she couldn't run it so). We have been playing it for roughly 2 months now. Sometimes ocationally when she isn't online or in Skype i would log into league and play a match or 2. And thats when the issues started. Due to playing apb at a much lower mouse sensitivity to make my aim better i wasn't used to the typical fast mouse speed i play with. on top of that i didn't feel comfortable holding my mouse anymore , my wrist feels uncomfortable asf when playing league. Sometimes i feel like either my mouse is not moving at all and other times it just seems to be moving alot (mouse acceleration turned off) mind you this has affected my farm/kiting and general movement all around. i play normal games but as a Season 5 gold 3 player now i can't even compete against silver 4 players as my normal game mmr is at that level. Adc is the worst role i can think of as kiting with Vayne(my 2nd most played champion) is basically impossible for me as i missclick constantly and then i feel like my mouse ain't moving and boom lee sin ulted me. 20 mins in and i still have 117 farm which is horrendous. I play on and off but i just don't feel like my playmaking is getting any better at all. Sorry for the wall of text.