I wanted to boost something that @Juli said. I learned to draw & paint in a very haphazard way, and eventually it began to show in my work. When I was struggling with fundamentals, I turned to DrawABox and it helped me IMMENSELY. Especially with thinking about plants analytically & working with organic shapes that have weight. I greatly enjoyed the assignments on DrawABox, since it starts with mark-making and continues on from there. The DrawABox community also introduced me to other resources that look promising for when I've finished brushing up on my drawing fundamentals (Ctrl+Paint).
If you suffer from perfectionism, and disappointment that your work isn't clicking in some way... it can be a wonderful thing to have hands-on instructor feedback every week. You can improve in leaps and bounds with directed feedback than you can from self-directed study. I took a few courses from CGMA, and really enjoyed the experience. CGMA Master Academy is NOT for the faint of heart, but it has a really great foundations program and it is welcoming of all skill levels. It's also expensive. $600-$800 dollars per class, and the schedule for any individual class is very compressed--8 week long courses, with weekly homework turn-ins. I took Dynamic Sketching I and II there, and found myself doing 20-30 hours of homework every week. It was an amazing experience, and I highly recommend checking it out if you have the time & money to do so.
For me, personally, SVSLearn so far has been the right mix of self-directed study & take-it-at-my-own pace instruction, so I'll also second the recommendation to check out the curriculum classes here.