Sometimes when I start to feel sorry for myself for no good reason, I think of the lyrics to the song "Rose Garden."
The lyrics I always sing to myself don't actually come in this order in the song, but they're the relevant lines for general bucking up.
I beg your pardon
I never promised you a rose garden.
Along with the sunshine
There's gonna be a little rain sometimes.
So smile for awhile and let's be jolly
Love shouldn't be so melancholy.
Come along and share the good times while we can.
Just replace "love" with "life" and you have a peppy little reminder that you're not really entitled to anything in this life, so you may as well enjoy what you do have.
And if you need a really amusing visual of this song and its most famous singer, please enjoy this 1973 performance featuring Lyn Anderson and her extraordinarily dated hair and dress.