You can lead a horse to water, but you cannot make it drink. The idiom is relevant in many facets. Individuals will talk about wanting to change and or make a difference, so they seek guidance and plan how to get there, and then often find failure.
Very simply, it comes down to execution. You want to complete a marathon and realize it will require consistent and deliberate training. You will not and cannot run the 26 miles from day one, yet a slow, deliberate build-up in activity will get you through. Every few days or week, you push yourself further. You build competence, and with that comes the confidence to do more. You feel great. Your coach or trainer can only give you the tools and support, but you are the one who is running. You could have the best Olympic coaches in your camp, but if you choose not to execute correctly, you will still miss the mark.
The notion applies to our behaviors. There is little that cannot change if we set our minds to it. Slow and deliberate. Like leading the horse, force will produce an adverse reaction, while slow patience may likely win the day.