Algebra is only going to become more advanced as the years of school pass and if one has not gained full knowledge of it at its most basic level, it will be impossible to venture on. You have to algebra one step at a time. If you do these steps over and over, your brain will start to adjust and you will like it.
Addition is the foundation of arithmetic. If you practice the above-mentioned methods, it will improve your subtraction skills too. Apart from these methods, make use of math work books. Write down each number in the box and then use methods discussed above.