"Some days I have a great voice and other days my voice sucks. What am I doing wrong?"
The first thing I would say to answer this question would be that you need to own your voice. I was taught early in my voice training that a common viewpoint is, "My voice cracks...my voice sounds bad today...my voice this...my voice that..." I tell my patients that they need to own their voices. Why? Because you drive your car to work, you brush your teeth, you dress yourself, you feed yourself and it should be no different for how you make sound.
We combine respiration (breathing), phonation (sound) and resonation (how the sound is amplified by our laryngeal cavity/tract) to achieve a voice that sounds like "us." We must learn how to manage these three systems efficiently for speaking and singing because we are all unique and we are always learning.
The second thing I would say to answer this question would be to take notes. Keep a journal of everything hydrating and dehydrating you consume for 3 weeks. Keep track of emotional occurrences and fights. Keep track of stress levels. Keep track of exercise. Keep track of how often you are using your voice and in what contexts. Then review this journal. Hopefully you are also keeping track of "good voice days" and "bad voice days" so that you are able to pinpoint exactly what was occurring in days leading up to the bad ones. It is important to remember that emotions play a huge role in our bodies. When we are upset, we often feel a "lump" in the throat. This is tension, so without realizing it we have tightened the muscles around our vocal cords.
Hopefully you are able to change your viewpoint and what you say becomes "I wasn't entirely focused on producing sound in an efficient way today."
You are your voice. Only you can control it. Take charge because no one else will do it for you.