Okay, since you stopped by the sketchbook I will critique a couple pieces for you if you want it. be warned. I will critique the shit out of them but be extremely constructive. There is a lot to say about any particular piece but o want you to choose one that you think you need a lot of help with.
The first peice of advice I will give you is this, and hopefully it will answer a few of your questions like,"why do my imagination drawings suck...etc.". Stop working from imagination. Its good for generating ideas., but you need a lot more observation before your paintings from imagination don't suck. Unless you want to be an abstract expressionist. At this stage in your learning you need to be more concerned with observational study. Once you finish that study, of figures, landscapes or whateverthen go and redo that exact same study from imagination. You are working the same brain muscles that you will need to use when you are good enough to draw from imagination. I skipped around a lot. Are you still using a mouse? Save up and get that wacom intuos or beg for it. Painting with a mouse is just well....a waste of time.
You are getting better. You need to know that solidarity in paintings and figures and anything with form comes from values. Solid recognition of value weight in composition and relationships. I hate telling people to read loomis but read loomis I didn't like being told that, just the other week when I asked a very amazing well known painterhow he thinks of Color he told me to read loomis creative illustration. Its like the Bible for fundamentals. If you have questions seek out people you like and ask them, they may never find the questions in your thread.