This is so definitely one of those things we all say but don't often do. Well I found myself giving this advice only yesterday and it has just popped back into my mind while watching a friends YouTube video.
So here is my example; When you are on a airplane and you are given the (important wording coming up) 'safety advice'. This includes information relating to an emergency where oxygen masks will be dropped from the overhead unit. You are 'told' to always put your own mask on first, before helping anyone else, even your child. The main reason they inform you of this is so you make sure your ok and able to help the next person.
In my mind there is a clear and obvious link to our everyday life's here. Yes your probably not lucky enough to be in an airplane every day, but making sure you are physically and mentally fit means you will likely be better placed to help others. So not only should you not feel bad for focusing on yourself sometimes. You should feel that it's a regular thing, maybe even a requirement.
Too often I think and I hear people express negative thoughts or feelings about spending some (often much needed) time on themselves. This should not be the case.
Remember put your oxygen mask on first!