You have a great attitude, to seek improvement through practice. Don't worry about how long that one remark held you back, we are forever exploring and learning new things. The pity falls on her and her remark, it's a portal to her inner self and her projection had nothing to do with you, she was either setting such high standards that whatever she did be overshadowed by them, or maybe she had such low esteem that it poisoned her from even trying something challenging, you on the other hand have talent and it seems you harvested it and take up the challenges.

I too used to draw exclusively landscapes and trees, I was really good at it and still am probably, but I remember we were sketching on large blue paper with black pastel a fellow student, at the time I didn't know that my work was worthy of prizes but unbeknownst to me my teacher submitted that sketch to some event, and I placed 2nd in a competition, something about the school commission board or something like that. It nudged me ever so slightly to consider my mentality which always was "not good enough". I didn't channel that into drawing though, but it definitely improved my self esteem, enough that I was confident in trying new things and learning a lot more, and today I couldn't thank that teacher enough for what she did, it was only when I was about to graduate that she told me I had placed 1st in one competition for some painting and 2nd in that other one. To me it seems that people that know how hard it is to art, have an appreciation for talent and the encouragement I received from that art teacher and the ones before her has always stuck with me, and helped me try new things like web and graphic design and 3D modeling, it paid of too, so much so that I atribute it to getting recognized by a recruiter at Pittsburgh Art Institute and invited for a meething, simply because of what I thought to be a mediocre website and some snazzy theme and UI I had done, which apparently blew him away, but being 16 at the time and with the circumstances of my mom passing away earlier that year, it overshadowed the encouraging gesture and didn't persuade me to follow that path, but had I not received the past encouragement from my art teachers for what then too I thought was mediocre, which looking back with a more healthy attitude was actually good if not great, I couldn't have explored design, web and 3D work, leading me to make all kinds of strides in those directions.

Art is not for everyone and I want to express that those sketches you shared aren't mediocre at all, you have a talent, and you have invested a lot of effort as it shows, don't be dishearten and too self critical, trust me, your work is great, not potentially great either, great great :)! I will follow to see how you're doing on your path and offer any help I can, thank you for sharing your story and your art, they are great!