Whenever you are tempted to say yes to a request that is in your realm of expertise but not really what you do, think of it as stealing someone else's best opportunity and their home.
I got a call one afternoon from a woman looking for a coach for her teenage son. She said she found me online and wanted to know how I worked. I quickly explained that I am a business coach not a teen coach and offered to share a couple of colleague's contacts with her who specifically work with teens.
I know precisely who I work best with. I learned the hard way that saying "yes" to poor fits dilutes my effectiveness and drains my energy. Embrace being the expert you are in what you do best, refer what and who isn't a true fit.
Focus on what you do best and what you love best and you will find that it flows more easily to you. Be willing to refer out to others what isn't your core strength and focus rather than dilute your efforts by stealing someone else's home.