You can never truly love anyone else until you love yourself.  Until you really love yourself, your love for others is purely selfish.  It is an attempt to fulfill the missing part of you with whatever you enjoy about them. Therefore, I cannot trust others who don't love themselves to truly have my best interests at heart.

 ~Danielle Marie Crume

Why do so many people struggle to put themselves first? Why does making a personal choice sometime result in a sense of guilt? Well, because social cues everywhere suggest that selfishness is ugly. And it can be. But there is a vast separation between making a choice that is best for you, and being selfish.
Selfishness is the number one cause of conflict, disagreements&& cold war.  James 4 verses 1 and 2 say this,What causes fights and quarrels, don’t they come from your desires that battle within you. You want something but you don’t get it.” 
"Everybody is really selfish-to a degree of course. But it depends on that degree as to how selfish a person actually is. Sure, we all have our wants, wishes and desires, and would like to have them met. But most of us have hearts big enough that we aren't completely selfish". 

Poor faith in oneself can stem from any myriad of problems. Whatever the cause, the main way to address issues of self-doubt is selfishness. That's right. We must learn how to be selfish- or love ourselves first before we can garner respect or even lend emotional support to others. We are more likely to be respected as well as have our opinions respected if we carry ourselves well and are confident. However, this takes time and much practice. 

There's nothing wrong about being selfish. Nothing.

Whatever you did/do, don't feel guilty for having to say no or admitting you are burnt out. The reason we strive so hard to maintain great relationships with our friends, lovers, and family members is so we can depend on them. We rely on them to pick up the extra slack, and be understanding of our wants & needs. Just like they rely on us to do the same. It's a give-and-take.

Above all  always remember that our life is too short . Don't spend your time doing the things you don't want to be doing (within reason).
Living your life to someone else's principles is just as STUPID!. Your top priority should be yourself. Making sure to live your happiest/healthiest life  and no, that's not selfish.
Acting in your own best interests, when done properly, isn’t a selfish act – it’s a way to ensure that you’re making the very best of your life, so that you can help those around you to make the very best of theirs.  ;)