I think discipline is very important in any job we do. As an artist we have to focus on so many aspects of it that its good to be disciplined all the time. However there are days when you wake up and you simply don't feel in a mood to paint.
At the beginning when I started working as an artist I was very hard on myself. I thought that its wrong to have days off. Then I realise that certain things cannot be rushed. Sometimes making a break is better. Coming back to painting with a fresh pair of eyes is the best thing I could do. When I take a break I try to go to galleries or spend time them with with friends.Positive attitude always help me get back on the track and bring new ideas. Longer breaks create some sort of a "build up" in my mind.. that needs to come out..and then I find myself woking even harder afterwords.
It helps to make a plan of all activities that need to be done. Planning is essential and helps with staying on the track especially if you paint for yourself and paint commissions in the same time.
If you struggle with motivation go for a walk and remind yourself why you have decided to be an artist. We all have moments when we need a break and you should have one as well. Don't punish yourself, just remember you are better when you are more effective.
My second big passion is dancing. Thanks to dancing I can completely switch off, forget all problems , issues, worries and sometimes I even get some inspiration. Maybe you should find another activity that makes you happy. Believe me it works.