I think it's good you're going to therapy. But you have to realize there's nothing you can do to make your girlfriend less depressed around Valentine's Day. Something terrible happened to her, and it's just always going to be a part of her life. Eventually she might get to a point where she can celebrate the day, but she'll have to get there on her own.

The other thing, is that when you say sometimes you wish you could be C, do you always feel like that, or only around this time? If your girlfriend makes you feel loved all the time except when she's remembering someone she loved dying tragically, I'd say yeah, that's part of dating a widow/widower. Now if it's all the time, then that's something worth exploring in therapy. To see if it's really the case or if you're feeling insecure.