You don't have a mouse mat, and you use a generic mouse, even if you had a gaming mouse it will be horrible on wood like that. (This is coming from someone who used to sit with a keyboard on my lap with the mouse to my side on the bed, and even botched an old t-shirt over the top of my desk as a mouse mat)
I can't imagine you are sat in a comfortable position with the height of the chair to the height of the desk, that won't help either.
144hz+ monitors help a lot, I had this one for ~ 9 years(i think), probably be able to pick one up second hand somewhere cheap(if you don't want to waste too much on gaming) as they aren't that much more than 60hz monitors nowadays.
If you have frame lag check what fps(tools -> time demo statistics) you are getting over a worse case map, then set a fps limit to ~ you average fps so the fps is more consistent making aiming a bit easier, I can get over 1000fps in some areas but there is no point letting it get that high if you average 400-600fps on a worse case map, that's why I just limit fps by netspeed 25000k(390fps) and if you have a weaker CPU limiting the fps will also help as UT is mostly CPU dependent and it will put less stress on it.
Also make sure your windows power plan is set to high performance when you are playing, you don't want your cpu going down to 900mhz while playing.
