First up I would invest in proper cleaning products designed for the job, tar spot remover will break down tar so it can be wiped off with ease rather than being rubbed/scratched off.

Also, remember that when you polishing or using t-cut, your dragging round grit and dirt and rubbing it into the paint which will cause more damage.

Maybe look at claybaring the car, it's quick and easy and cheap and will help make sure the paint is free from dirt and grit before you being to polish the car.

Have a read over some of the guides on here for the processes of washing and polishing your car the best way.

Maybe worth looking at spending the money and investing on having a professional do some paint correction (about £300) so that you have a good base to work from and then get some nice products that will keep the paint in top condition.