Why am I getting constant crushes (in a relationship)?

I'm in a relationship with a man for almost 5 years. For the first 1.5 years he was great, but struggled with depression after losing a job (still got on his feet). I would say I know him more as having depression than him without it. I am helping as best and supporting him... but I have found that since about a year ago I keep getting crushes (maybe 3 so far), on men I have spoken to. I of course never act on it or mention/flirt. It just stays in my head. It's like a weird spell that lasts maybe a month or so... It sometimes makes me cry and make me incredibly sad at night.

Why am I getting constant crushes (in a relationship)?
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