Feeling sad about past relationship

My ex and i are no longer together largely because i want children and he does not. I am happy about the decision to move forward in the direction of my dreams, even if that means without him, but i cant seem to shake the sadness about it. There was a point that sadness felt like a useful negative emotion and so i sat through a lot of it, but it no longer feels useful, as its taking up A LOT of my headspace as i try to move forward. If i work on changing my thoughts to not feel sad, is that resisting the emotion?