Why can’t I get over my ex. Plz help.

So me and my my ex bf have been broken up nearly seven months now, he broke up with me back in October, three days before our anniversary.

He was planning to go into the Navy and he said that because he didn’t know where the next year or two or three would take us so he thought it would be best to break up. I didn’t agree but understood to an extent but was willing to fight for our relationship. After all we had already discussed marriage and having a family together. He left for basic training today and I saw a post from him mom on Facebook and it got all these emotions stirred up again.

But not too long after we broke up he started seeing another girl which my girlfriends informed me of on Valentine’s Day (perfect timing right?) but anyways why can’t I move on from him? What is keeping me hanging on?

Also if anyone has any advice on moving on and getting over an ex, I’m all ears.