Low sens isn’t great if you are not used to it, you need to play at a new sens for about 4-7 days to start seeing improvement.
Use the exercise in this video to find your perfect sens. Then if you want lower, lower incrementally like .5 per week. Small changes are better than big ones when it comes to muscle memory.
Also make sure your settings are optimal for fps (I posted mine a few days ago)
(in the screenshot I have local reflections on, you want this off, unless you can afford to trade fps for golden gun shine 
)
Also make sure your input lag is as low as you can get it.
https://www.reddit.com/r/Competitiveoverwatch/comments/4vz5sh/cut_my_input_delay_in_half_and_loving_it/
Then just practice. Not just against bots. Ana bots are ok up to a certain point.
I never used OSU , I had good aim before this game, but struggled at first when I played this, setting tweaks are half the battle.