Some days are just "down days". Time to stop the running, doing, and achieving to just BE. Do simply what has to happen and spend the rest of the time enjoying what is.
Snuggle your kids, enjoy your new baby, wrap up in a soft blanket and soak it in. Truly feel the sensation of parenting - small hands in yours, little breath on your skin, young voices in your ears all covered in love.
Also feel the hard parts - the passionate preschooler tears, the needy newborn cries, and the depth of exhaustion from being a true and complete full-time care giver (because that is what parents truly are).
Taking the time and feeling this parenting completely is what will keep you going/doing/achieving when you need to. The memories of sweet baby breath can sustain you in the wee hours of the morning after the millionth wake up of the night. The feelings of love and joy will be what balance your fears and frustrations of parenting when you most need it.
As it is said, "the days are long but the years are short". The small hands grow, and the desire to snuggle and be held fades.
Take advantage of these long days, and have some "down" ones. Hold on to these memories, good and hard, because they are what you take with you long after the short years.