Everyone goes through slumps, Kate. The obvious choices I'd suggest
1) Power through. Set yourself homework of writing N songs a week, where
N may just be one. It's OK if they're lousy and never get shown to
anyone else; the goal is to exercise the skills and to stop
self-criticising prematurely. Practice makes better, and helps get you
out of the "I can't do it (anymore)" or "I can only do it when inspired"
2) Don't worry about it. It's been a decade or two since I wrote
anything serious, spend significant time practicing, etc. I will at some
point, but I flat-out refuse to stress about when. There are many folks
who have a burst of creativity followed by a dry spell, no matter what
the field; if you're waiting for the next one, accept that you're waiting.