Whenever I try opening an existing project, or starting a new one, it doesn’t load past 45%
As explained in this question, you probably just have to wait it out. That post says 2-30 minutes, but in my experience it can take hours with a slow computer and a large project.
Also as explained in that post, you can verify that it is indeed processing by opening task manager (Ctrl+Shift+Esc) and seeing that CPU usage is very high.
If you just want to get your project loaded faster, you might try taking pieces of it out, and bringing them back in in smaller chunks, though I haven’t tried this, and thus can’t speak for its reliability.