Feeling abandoned

I recently had a child with my fiancée. Or at least we thought it was mine. Turns out she was already pregnant before we got together. She decided to tell him and got a dna test before even discussing it with me. I found out on accident and she acted all sad. I asked questions like did you know? You cheat? She said no to both. I asked if she loves. Said both and she’s confused. I’m so confused myself. How can you be with someone for almost a year and love more than one. When I do talk to her she’s distracted.. I feel he’s trying to win her back. So now if I lose the chance of having my biological first born and my fiancée I’m not sure if I can take the agony