First, don't be discouraged by an "F". Fs happen for a variety of reasons unrelated to talent or hard work. I understand the desire to save paint as a student, but diluting the paint will hinder solid coverage. You may need to apply two coats to get the kind of flat solid you seek. Be sure to let the first layer dry before applying the second. Masking tape may also be useful.

Nice painting, and coloring for contours. It may be helpful to look at the rib cage to breast transition in an anatomy book or two. The breast emergence from the rib cage has the the ribs looking a bit more concave under the breast than they should. Think about rounding out the chest a bit, chest pectorals on top of ribs, the breast tissue on top of pectorals. I hope this is helpful.