Hey Jane, I am a guy, but incidentally I had a reverse situation with my girlfriend. She wanted to have sex multiple times a day but I could only do it once every 2-3 days. During that time with her I suffered from depression, didn't exercise much, and didn't have the best diet.

Things that can affect sex drive:

-Stress, depression, anxiety
-Diet
-Suppression of endorphin release

Things to help:

-To mitigate stress; find good friends and people to be around. Cut out those who negatively affect you. Social situations can significantly affect they way we feel about ourselves.
-Diet (eat a balanced diet i.e. fruit, vegetables, nuts). Cut out more unhealthy things or simply just eat less of them.
-Exercise (exercise releases endorphins which are also released during sex). Exercise significantly helps with a lot of functions in the body, brain, and mind.
-Emotion regulation; find things you like doing (i.e. if you like art, draw; If you like TV, find a good show and cry during the tough parts; If you like music, listen or play some music). Make time for yourself.
-Know yourself; you're justified in feeling a specific way. Maybe there is no specific right thing to do, but give yourself time. It will come back. Hopefully he can understand that too. It's not just you, a partner should understand the limitations of another and be understanding.