I believe that we are all hardwired to be selfish at the core. Recognizing it and rejecting it as a modus operandi, though, is a very good and mentally sound thing.
The fact that you tried to show happiness for your sister-in-law's pregnancy even though though it was killing you inside shows excellence in your character.
Definitely talk through these issues and pour your heart out in prayer--but please know that you're not some kind of monster for having these bitter feelings. You ended up conducting yourself nobly.
Anyway, though you are right to fight selfishness, there is absolutely nothing wrong with loving self-care. Treat yourself like a queen this week (and for as long as your emotions are raw).