Thanks for your question. My name is XXXXX XXXXX I'd like to help you out.
I'm terribly sorry to hear about what you've found out about your partner. It must be especially difficult since it seemed like your relationship together had been progressing by moving into the new home and going on holiday together. While I can't explain why exactly he is not with this other woman, it is certainly possible that it has to do with his inability to have kids, or provide the life for her that she really desires.
Either way, it sounds apparent that his relationship with her has never gone away for good, or if it had, he has rekindled it again. Knowing that this woman has been an issue in several of his last serious relationships, it does make it hard to believe that he would end things with her completely. However, a lot depends on your willingness to give him another chance, and if you confront him on the issue and he seems genuinely willing to work on your relationship and perhaps also attend couples counseling together, it is certainly possible for him to change. You may be able to tell a lot by how he responds once he is confronted with the information that you have uncovered.
The larger problem may be whether or not you feel like you'll ever be able to trust him again. Based on the texts, it sounds as if this has been going on for some time now. If you feel like this will continue to be a problem, or that you will always be on guard from this point forward, then it is probably time to call it a day and end the holiday. If that is what your instincts are pointing towards, I think that is important to listen to. In that way you can get some space and decide exactly how you want to handle this moving forward, and whether or not it is possible for you to stay in this relationship.
I know you're caught in a tough position and I definitely wish you the best with all of this. If there's anything else I can do to help just let me know.