Your approach is very good. Trying to learn a language by embarking on a project is an excellent idea. Just keep in mind that a project, even a "simple" one like a text editor, can be much larger than you expect at first, and be prepared for making a lot of false starts and having to rewrite a lot of code. After all, that's the point of exercising.
What is not clear to me is why you are not satisfied with scintilla but at the same time are looking for an alternative. I would recommend to start playing with it and getting a first taste of what a "complete" project looks like (for some definition of "complete") and then you will be in a better position to appreciate the strengths and weaknesses of scintilla or of the other various libraries.
On the side of learning C++, since you are asking for suggestion, I can recommend a book: Accelerated C++ by A. Koening and B. Moo.