For what it's worth, I've always struggled with the same thing. Push through. It's a skill of itself to work even when your motivation suddenly vanishes, or when your emotions and insecurities betray you. You'll get better at it. Personally, if I make myself draw for 15 minutes, I nearly always get my inspiration back (although it might be inspiration for a different piece). I'm currently living off my art and very satisfied with it, but it did take me years to get here. Push through.
When you have no motivation and no inspiration, do mindless studies to get going. When I'm exhausted, I sometimes trace anatomy from photos just to learn new shapes, or do quick ten minute master studies by copying landscapes. It works perfectly and I always end up drawing something original as a result.