Getting over an ex

How do I get the feeling that I miss someone I chose to broke up with? I think it's just Christmas or the fact I'm in quite a bad place at the moment but I only knew/dated my ex for a month and then chose to break up with him in November. This is sort of just a stream of consciousness but he was so lovely and attractive and we did have similarities but I know I wouldn't want to be back with him because we weren't compatible. The break up was sort of an accident (I didn't plan it) but I stand by it. I've got with people since then but I don't know what's going to get rid of this feeling of missing him other than dating someone new.