my fiancé suddenly wants space

My fiancé has always told me how perfect I am for him, how there are no differences, we are on the same page with everything in life/goals etc. We became engaged fairly soon in our relationship. We both ended relationships soon before we started dating. We were ttc and were planning our wedding. Lately I have felt like he has been distant from me emotionally. I have asked him multiple times if he was okay and he has given me reassurance that him and I are perfect. A few days ago he mentioned that he might not want to have children ever, days after we bought ovulation tests and were trying to convince. Talking some more he admits he still is in love with his ex and it is messing with his head. He says he needs to settle that part of his mind before he can commit to me 100%. I was shocked when he said this. He says he does not want her back, he just needs to end that part so he can figure out if he can be with me or not. I am completely heartbroken. He says it's not coming down to me and her, but me or him being alone for awhile. He is planning on talking to her to settle what questions he has in his mind and that should give him clarity. I just don't know what to do. I was in the process of moving in with him and how do you propose to someone then suddenly get cold feet? I am very depressed and not sure how to move forward. Do I trust that he is talking to her and sorting it all out so he can be with me and make our relationship work? I understand we jumped into our relationship and he might just need more time to heal, but do I wait for him to do that? What are reasonable expectations for this? He says we are just "pausing" our relationship until he sorts it all out. I hope we can be together and continue to work and move forward. I have never trusted or given someone all of me, especially so soon in a relationship and I'm terrified I'll never have it again.