Use yourself as reference when you're doing the mapping exercise from Keys to drawing or find a little statue and place it in front of your window on a sunny day.You can see I used a Shrek statue and then I made a self portrait while looking at a window in my living room under the light bulb at night.I was surprised how powerful the technique was since I didn't do any eye measurements to get the proportions right and it came out really,really accurate.
Your best bet is going to be a statue,preferably a white one because you can block the shades that fall on it better but grey or terracotta will work too.You're either going to have to find a statue in the park,buy a little one from an art store or make one yourself from clay which will help you when you study anatomy and proportions too.
It's good that you do different sorts of studies but I think you're not doing enough of them.You literally need to fill pages with studies and repeat them over and over until you're doing them from your own memory.
Also,try to work from black and white photos with high contrasts.That's how I improved at rendering and you've seen how those portraits look or just place stuff under a lamp and start shading.
Now get to work!