Having a lot of trouble getting motivated to learn lines

I've been cast as understudy of one of the roles in the campus production of The Importance of Being Earnest.  As I understand it, there's an "understudy run" which is essentially a tech rehearsal on the set, with no costumes or only bits (e.g., skirts).  We don't get an actual performance, and I don't even know how much rehearsal we'll get before that run.  We're all supposed to be off book (i.e., have all our lines memorized) by Thursday, but I'm finding it really hard to make myself work on my lines when I haven't been to a rehearsal since the read-through (which I wasn't even required to attend).  For all that I'm getting the daily emails, I don't feel like I'm part of the production; it feels very weird.