Hope is gone

I'm 41 and have been in a relationship with an amazing man for the past 6 years . In fact so amazing he chooses to stay with me knowing we can't have children due to a medical condition. It has put a strain on our relationship both physically ,and emotionally. Today a new life enter our family my cousin that said she would never marry or be a mom is now all of what I've always wanted . I feel so horrible that I'm jealous and hurt when I should be happy for her. I am happy for her I'm just having a really hard time accepting my new reality . How do I heal from this ? How do I get him to understand what I'm feeling ?