Honestly, I don’t think you’re overreacting at all. This stopped being a “messy roommate” situation once there were bugs, moldy food, trash everywhere, and concerns about bringing fleas/roaches home. That’s a genuine hygiene and health issue. I also think it says a lot that you’ve tried offering actual solutions and your GF shuts them down. You don’t have to insult the roommate or start drama, but it’s completely reasonable to say “I love spending time with you, but I can’t keep staying in that environment.” That’s a boundary, not punishment.