Discipline is not something you are born with. It is a mental muscle that needs constant training. Like any other muscle, if you don’t use it, it will weaken. It takes more than just wanting to be disciplined. It is an ability that must be developed and tended to. If it appears easy for some, it is because those people have cultivated it for a very long time.
By practicing your discipline, you will notice results in your works out as well. Studies have shown that people who do small mental tasks at work, increased their work out stamina.
And remember no muscle gets into shape immediately. You have to build it and maintain it. All your choices affect this muscle. When you make an excuse, you are hurting the muscle. This doesn’t mean you should expect to be perfect and never slide, but it does mean you understand that it is important and you continue to build that mental muscle.