
It’s valid for you to feel upset, but it’s about what you choose to do with those feelings. You may be lesbian, but you also need to understand she is bi. If you can’t handle that, that’s your issue that you need to work on, not hers. So you can either go talk to your therapist and find a way to get over it, or if it’s a dealbreaker for you then end things and move on, but you can’t stay mad at her for being who she is as a bi woman.
I heavily disagree with your last point. As a lesbian, I feel like you should understand why it’s different. I’d be upset regardless of gender, but men add another layer because of the fundamental differences between us. It’s like a straight woman feeling insecure about her boyfriend only dating blondes, except I can’t dye my hair. I can’t grow a dick or become a man, and that makes it uniquely hurtful because it’s something I can never change about myself
But it’s you who needs to figure out how to deal with your insecurities. She’s in a relationship with you now, and you’re jealous of someone she’s no longer seeing because they did something back before you were exclusive. Did she lie about it? Or did she just not tell you until now?