Think about drawing from reference and drawing from imagination as two separate muscle groups. When you draw from reference, you are exercising those muscles and they become strong and flexible, but like the guy who only works out his upper body in the gym, your other muscles will stay the same or atrophy. Suddenly you look down and your draw-from-imagination muscles look tiny compared to your giant draw-from-reference muscles. It's important to practice drawing from imagination even though you know those drawings will not come out to your standards compared to drawing from reference. It's a tough pill to swallow because nobody likes making drawings they aren't proud of, but you've done it before to get to this point, and you can do it again! Just my two cents. It will be difficult at first but you got it :)

I also found those boots very difficult to draw