What he is saying within this quote, is that your mind is constantly searching for evidence to support what you believe.
If you believe that life is hard, or motherhood is hard, then that's what your mind seeks to find in your life. When you keep telling yourself that motherhood is hard, then this is where your attention lies and you look around at what's happening in life and your brain will highlight all of the situations that are currently aligning with that belief.
You will start to see all of the dishes that need to be done, become really sensitive to the noise in the house, feel resentful for the demands that are upon you and the lack of time out you get, you will feel resistance towards the behaviour of your kids' tantrums, backchat or unruly behaviour etc. and motherhood will continue to be as hard as you believe it to be.
This is how the brain functions. There is nothing 'wrong' with you or your life. Your brain is doing exactly what it has been designed to do and is operating under the instruction of your belief system (largely set up from your childhood). You are not better or worse than anybody else and if you feel like you are, it is simply because you have beliefs about yourself and your attention is directed towards finding the proof that supports those beliefs.
Think about the last time you looked into buying a caravan, or a boat, or a new car. How many times did you go about your day and see those exact items all around you. You find it amazing that all of a sudden you are seeing all these Toyota Landcruisers everywhere!
But what's actually happening is that Toyota Landcruisers are simply where you have your attention, so this is what the mind seeks out to find, just like if you hold the belief that motherhood is hard.
What about if you believe that you are not good enough as a mum, or even as a person? Your mind will seek out all of the evidence it can find of you being not good enough. It will seek out all of the mistakes that you made and the times where your 'performance' was less than perfect and you will say to yourself, "see, I told you I wasn't good enough".
Does this really mean you are not good enough? No, it's just that you hold an incorrect belief (that was also most probably set up in childhood) and this belief is unconsciously directing your attention to search for proof of this belief. This proof then starts to look like your truth. Believing that this proof is truth, then further reinforces the belief that you are not good enough and you sink further and further into self-hatred, because all you ever see is evidence of you not being good enough.
What if you hold the belief that I'm not liked, or my relationship with my partner is dying, or I'm not getting enough time out, or my kids are always being naughty, or I'm a slave, or the many other repetitive comments we make to ourselves on a daily basis?
That's right, your mind will seek out every situation it can find that supports what you are saying and reinforces these beliefs further inside your mind, making you feel worse and worse.
The trick to changing all this is to first become aware of these thoughts and then to deliberately replace them with something different. You need to consciously redirect your attention to the exact opposite of what you had been thinking when you were feeling low.
Becoming aware of what you are thinking creates 50% of the change that you are willing to make. Most of the time we have this relentless chatter going on in our minds that we have trained ourselves to tune out. In order to get a really good insight into why you are feeling so low, or why you are finding motherhood (or anything else for that matter) so hard and challenging, you need to tune back into that chatter and find out exactly what you are blabbering on about in there.
You will be very surprised by what you are actually saying to yourself.
After you have become aware of what you are saying, you can consciously change it. If you have been saying "I'm not good enough", or statements of a similar nature, then you might change it to, "I'm always doing the best I can", or "In every moment I am learning new things and adding to my knowledge or life", or "I'm always good enough".
Whatever thought you would like to hold, you will need to repeat it over and over again in your conscious mind and you will automatically start to search for evidence of that being true, and you will start to feel differently about that situation.
"I'm always doing the best I can" will have you looking at what you did achieve instead of your shortcomings. It will help you to see that at the time you were acting the only way that you could've, with the current abilities and priorities you had in that moment."
"In every moment I am learning new things and adding to my knowledge of life" directs your attention to look for what you did get from a situation instead of what you didn't get and how what you didn't get makes you not good enough.
What about your relationship - start saying to yourself, "I have a supportive, loving and aligned relationship", and you will start to see all the evidence of your relationships strengths, rather than it's weaknesses.
"My kids are learning how to behave in the world", rather than the belief, "My kids are always being naughty", will help you see opportunties to help them learn how to appropriately behave in life, and help you to see that their behaviour is developmental, not a deliberate attack on you.
What you are doing here is literally retraining the mind to think differently. You see it's not the events that are making life hard, it is your perception of those events. It is the mind chatter that you have about those events that create our stress and by being conscious of these beliefs and changing them, we effectively change what we think about. When you do that, you search for evidence to support these new beliefs and you start to see life very differently and therefore feel differently.
This is what Wayne dyer means by - "When you change what you think about, what you think about changes".