Relationship advice? Help!!

So I was with my ex for 4 years and have a 1 year old son to him. I loved him dearly and we broke up when my son was 3 months old. He is the most amazing guy but I thought the grass was greener on the Otherside and really wanted to be with this other guy. Now I'm in a relationship with the other guy and my ex has a new gf. I am still so in love with him but he seems to have moved on and now I don't know what to do?? Hellp