It is with much certainty that I can say you either have already heard, or will hear in the nearest of futures, the phrase "Enjoy every moment, they grow up so fast." It is a lovely sentiment and I know the people who say it mean well and are saying it with love, but please let this serve as your permission to ignore it. You do not have to enjoy every moment, because let's be honest, not every moment is enjoyable. We are talking about children here, and even though they are yours and you love them with every fiber of your being and you would die for them, they are children and they can be the worst at times.
You don't have to enjoy the meltdowns in the "terrible twos" or the temper tantrums thrown by your "threenager." Or when your 5-year-old suddenly starts lying about everything and talking back to you when all you asked him to do was finish his chicken nuggets or put on his pajama pants a little bit faster than frozen molasses.
Speaking of chicken nuggets, you don't have to fondly look upon dinner time when you struggled to get your kid to eat anything other than chicken nuggets or when you other kid would eat just about anything but if you wanted her to eat more than two bites, you had to feed her yourself, lest she just play with her fork.
You don't have to soak up the moments when your kids are "playing together" and all you here are "No!" "Stop it!" "That's mine!" and all other instances of yelling and crying and doors slamming.
Or potty training (enjoy it when you can trade in the -ing for -ed). Or teething. You don't have to enjoy those moments when your baby is crying and there isn't anything you can do about it except pray that the damn tooth will just break through already.
I read an article once that said this: "There are plenty of moments in motherhood that quite honestly, I could do without...the ridiculous and nonsensical meltdowns of toddlers, siblings bickering at each other over every little thing, losing my mind and showing my ugly side to my children... We've created a myth that if we're not loving every single moment, we're doing something completely wrong. " (Sorry, I really don't mean to steal this, but I honestly don't remember where I found the article or who wrote it.)
There are going to be a multitude of wonderful moments with your children that you will hold onto, moments you will cherish for all time. Those are the moments you will look back fondly on. Those are the moments that make the not-great ones worth it. Enjoy those moments, and if you have bad ones, even phases of bad ones that seem to never end (spoiler alert: they will), don't be hard on yourself. Put the kids to bed, pour yourself a glass of wine, and look forward to tomorrow.