How do I(M19) better support my gf(M18) when she’s feeling down?

About every month(a bit before and during her period) my girlfriend gets pretty sad. Usually it’s about her own insecurities like weight or attractiveness. I try to comfort her and be there for her but it feels very in one ear and out the other, unless it’s something negative and then it sticks with her. For example she has, objectively, gained some weight from when we started dating. However before this (I think, I’m not a doctor) she was underweight so now she seems to be a much healthier weight. When she asked if she gained weight I said “I think you gained a little weight but nothing unhealthy” and she seems to have only heard “I think you gained weight”

Also I can’t just say “You didn’t gain weight” she really doesn’t like lying even white lies
I’m not going to break up with her, everything else is amazing
Therapy is currently not an option but I am trying to make it an option and will turn to that once it’s available.

How do I either phrase my words so she doesn’t focus on the negative part, or what can I say before to get her to not focus so much on negative things?