You should love your person as much as yourself. But you should not love your person more than yourself. There is a significant difference.
If you love your person more than yourself, then you are going to end up losing what they love the most about you. You are going to make sacrifices to please them. You are going to trade your passions for theirs. You are going to become a completely different entity than the one they decided to spend their life alongside.
They would not want that for you. They would not want you to feel like the only time you are alive is when they are by your side. They would not want you to feel like life is meaningless when they are away at work or with friends, leaving you alone.
Your person wants to mean something to you. They want your attention. They want to feel wanted. But they do not want the responsibility of being the only thing in the universe you care about.
You cannot become so distracted by loving them that you forget to spend time loving yourself. You are allowed to have alone time. You are allowed to pamper yourself. You are allowed to spend at least a small chunk of your time making sure your mental and physical health are up to par.
Never say you are too busy for you. Think about how often you go out of your way to do something cute for your person. Count up all of the times you ran straight to them when they needed you, when you answered the phone even though you were tired, when you pushed aside everything else in order to make sure they were okay. If you could make time for them, you can make time for yourself.
Your love for them should never outweigh the love you have for yourself.
It is not selfish to take care of yourself. Your person does not want you to crash and burn because you are more focused on what they need than what your own body needs. They do not want to be the reason you are neglecting yourself.
Besides, if you stop taking care of yourself, then you are not going to have the energy to take care of your person. You are not going to be able to support them the way you want.
Remember: You can love more than one thing at a time. You can love him. And love yourself. You can spend time making him happy. And spend time making yourself happy.
There are certain things you might have to sacrifice for your relationship, but your relationship with yourself is not one of them. You do not want to lose your identity by getting too tangled up in theirs.
Even though your main goal in life might be to make them happy, you cannot expend all of your energy loving them and leave nothing for yourself.
You have to put as much effort into your relationship with yourself as you put into your relationship with them.