My bf is cheating on me with ex

I don’t know how to feel. I’ve been dating my bf for a year and I found messages with his ex gf. He never responded but I saw that she sent him nudes. And I noticed she sent a couple of photos and she’s a busy woman. I know he loves that. And he said that he doesn’t love her but I can’t help thinking of her breasts. She’s a bit chubby but her breasts are huge and she was telling him how she couldn’t get wait to get married. Do u think he’s secretly messaging back. I secretly am attracted to her body and her breasts. And I think I’m turned on if he was cheating on me with someone with such big tits. They were like G cups. I’d like if someone describes what they’d do if they were the gf, I want to hear all the thoughts that would go through one’s minds if their gf didn’t know.