How does everyone go about deciding on what to take, and what to leave behind

I have 12 short sleeve shirts, going to take 7 of them; 28 t-shirts and going to take 18 of them; 4 shorts and taking 3; 4 pair of trousers, taking 2

Right now deciding on the ones I wont be taking, and putting them away, but still have to narrow somethings down, on one hand want to take that t-shirt, but also want to take the other one, but one of them has to be left behind

I know when it comes to me packing my case, more likely to take things I dont need, and if I go with a 15 kg allowance insted of 20 kg, Im going to have to remove alot more clothing to make the 15 kg