Taking care of yourself is not selfish, it's being smart.

 You have to take care of yourself first in order to be able to help those around you when they need your help. Your not being selfish, your being smart. It may be hard to say no, but if you know you can't, then you have to say no because it is in everyone's best interest. You can throw someone a life preserver if they find themselves on thin ice. But you can't go on the thin ice to help them and put yourself in danger. It's important to be helpful and optimistic, but you have to be realistic about what you can and cannot do for someone. Taking on more than you can handle means not everything will get done. You can only do what you can do. Doing what is best for you is what is best in the long run for everyone. Especially now during a pandemic you have to be careful to keep yourself safe so that you keep everyone around you safe. Doing what is best for yourself is not being selfish it's necessary to ensure that you will be able to help others when they really need your help in the future.