You don’t have to do math in your head, but you should understand how to do basic math and algebra on paper. The more you understand, the easier it becomes to do the math in your head. The best way to get good at math is to practice and put time into it. Make sure you really understand a concept before you move on to the next one, which could mean doing extra work or getting tutoring. Much of math builds on previous concepts, and if you move on without fully understanding something, you get lost very quickly.
When you work problems on paper, make sure you show all of your work. Students seem to want to skip this step and just show the answer, but it’s important because if your answer is wrong, you can’t find your mistake. Also, rework problems that you get wrong, so that you fully understand why how to get to the correct answer.
This will be a lot of extra time and work at the beginning, but as your skills build and your foundation becomes solid, it will take less time.