In times like these (come on, even those of us who see this as "no greater joy" still have times like this, right?...or is it just me?...are those crickets I hear chirping?), I've found great encouragement in a few things:
1. Admit it - to your spouse, to friends, hey, even on (dare I say?) Facebook. I did all of those things recently. My husband is amazing and knows how to refresh my spirit. And all it took on FB was a status update: "I could use prayers today. Having a tough day as a mom/disciplinarian." That alone lit up my wall with encouraging comments and prayers of friends that I know are lifted me in prayer. It also welcomed inbox messages from people who otherwise wouldn't have ministered to me (thinking I otherwise have it "together" all the time).
2. Pray - which is "admitting it" to God, but it is also a way to soften your heart towards your children, because you do want to mold them into God's image and into turning their hearts towards him.
3. Read your Bible. The ultimate in true godly counsel!
4. Good music. My 15 m.o. son figured out how to find music on my iPhone long before I did! The other night, he pulled up one of my favorites (okay, he's not really a genius, he didn't know it was a favorite, but I do think God was working through my baby's fat little fingers at that moment). Jars of Clay "Boys (Lesson One)" - it speaks of a parent handing down lessons of great worth...and the reminder that all this (growing up, parenting) goes by all too quickly...
Sara Groves also has a couple of songs that I'm trying to keep in my heart and mind: "Prayers for This Child" and "Song for My Sons" (the first being a "prayer" and the second being similar to the Jars' song for its "advice").
Hope that brings you some encouragement to keep pressing on!