The brain likes familiar routines — we feel safe doing what we know, and unsafe when facing something different. When we are deliberately trying to change our ways, there can be a surprising amount of resistance from within.
Our thoughts are like a wild meadow. If you have walked a certain path several times a day for years, it has gotten pretty easy. You have cleared a path for yourself, and there is not much resistance from the nature anymore. However, if all of a sudden you decide to walk into another direction on this wild meadow, you are going to endure some serious resistance.
It will take you longer, and there will be weeds and bushes slowing you down. The beaten path, the one you know so well, seems easy and inviting compared to the new one.
This is what happens within us when facing something new. Our thoughts would rather slip through the known path than venture out into the wild. To make serious change happen, to stretch ourselves, we have to give it time. We have to be uncomfortable and trust that the new path, no matter how bushy at first, is going to eventually lead us to better places.