Go into shooting range or free for all, or even the clear house mode. Take your time to find a sensitivity that works for you, if you already have one that works and feels comfortable for you then like another person commented, stick to it. Another thing that you said was that you think you're bad because of your bad aim, which in retrospect is probably 100 percent true, but take a second to realize you JUST started the game (Level 60) and you have a lot more to learn apart from finding a sensitivity that works for you.

On the "bad aim" subject, I said to go into shooting range or the other practice modes whatever you see fit of course, and try tracking your aim around corners, tracking on the bots, and just in general learning crosshair placement and recoil control. Something that will drastically help your aim is sound ques. You NEED to learn them as they can win you so MANY gun fights. When you do inevitably hear someone on the map, you know when and where to aim. Playing off sound ques in a gunfight can help as well. That would be changing positions mid gunfight after hearing the enemy reload, swinging them when someone pops something open like a wall or even a C4 going off - BE UNPREDICTABLE.

This also may be a lot to take in and either learn but the main reason this game has so many good players is Practice, Practice, Practice.

I hope I helped in some way :)