Finding work life balance begins with learning to say no. Stop doing everything. Say no to every task that is non-essential. The way you define your priorities is by saying yes to some things and no to others.
That means when your daughter is anxious at bedtime because of a scary movie she saw or goes through a bout of separation anxiety at daycare dropoff, you take an extra 15 minutes to cuddle and reassure her before leaving the room. But when it's your busiest time at work and you're asked to chaperone a field trip, inquire if you can help at school later in the year.
If your boss drops a mammoth, urgent project in your lap, ask which other tasks he'd like you to delay in order to get it done on time. Don't pull multiple all-nighters to finish the extra work on top of your regular job.
And please, lower your standards so you're not killing yourself to have a perfectly clean home, lavishly planned parties, from-scratch cookies for soccer practice or whatever else is important to other people and not to you. You'll never find work life balance unless you let some things go.
You simply cannot be everything to everybody. It's up to you to draw the line when you see it's hurting your children or your own peace of mind. A good test is to ask whether the question you're wrestling with will matter in 20 years. Generally speaking, the time you spend with family or on genuinely career-building projects will pass muster -- but the extra three emails you can answer at 11 p.m. will fail.