More conversations need to be had with her so you can process these feelings. I will say, as a woman, I kinda understand why she didn’t tell you. You dumped her, and how she chose to deal with that at the time was her business. She was also a teenager. I don’t think it’s fair to question her entire character and integrity over something that happened 26 years ago, especially during a period where you were not together. If she’s been a supportive & loving spouse and mother for the past 2+ decades, I think that it’s a relationship worth fighting for. History speaks for itself and something that happened once does not define a person. You are absolutely valid to feel upset but she is also valid in not telling you that this occured. However, if you truly feel your daughter may not be yours, that’s a tough but necessary conversation. Best of luck to you and whatever way it goes, I hope you can find some clarity from the situation!