Learning to read music is akin to learning a new language. You have to constantly reinforce what you’ve already learned, and build your skills from there. If you take a break in the learning process, you will regress rather than move forward. There are music theory books that can compliment your regular harp lessons, or “Notespeller” books that go along with piano books that may be helpful as well. I think one of the best ways to learn to read music is to internalize (read the notes, identify them, etc…), and then externalize your knowledge by writing your own music on paper or in notation software. This way you are directly applying what you have learned and forcing yourself to use your knowledge in an external manner.
I think many of us just learned how to read music at the same time we were learning to read words, so it’s just hard wired in to our brains since childhood in the same way English (or whatever first language we learned) is. The good news is that adults can learn very quickly, but you just have to be persistent and patient.
Learning any new “language” isn’t easy, but it’s not difficult either if you try and make daily progress on it. As an arranger, I’m always reminded of the challenges of reading music whenever I have to arrange for viola, which uses a “C” clef, and it makes me more sympathetic towards my students who are just starting to learn to read music (and for the young ones, words as well!). You will get there, but you have to be patient, and expect moderate, steady progress.