Self-discipline is essential to achieving success. Sometimes we have to do things we really don't enjoy but they have to be done to reach goals. Self-discipline really is the way you motivate yourself to do the things you'd rather not do.
Guess what! You're not alone. We all have days when we can't motivate ourselves to do what we should. This leads to feeling bad about it, then we get really hard on ourselves which makes us feel even worse. Getting your self-discipline under control will help keep you from feeling bad.