People who learnt to prioritise themselves have usually been through a lot of pain, to have finally learnt how to take care of themselves. It's not easy. But you will reach there too my friend. For that, you need to find a balance between being selfish for your own good and knowing when not to be selfish to help others. Also, sometimes if you don't feel like doing something, don't say yes because others are saying yes. If you want to prioritise yourself then you need to learn to stand alone.