How to get over seeing ex with someone else

I was with my ex for a few years. First love and all that. We broke up last year and have been on/off ever since. Stupid, but it's become one of those cant live with him, can't live without him situations. I still love him to bits, but it's usually when he's not around that I come to realise this. Our relationship is not what it was, I know now that it will never be fixed because we've tried so many times. We broke up for the final time a few weeks ago. I know it's not going to work out so I'm trying really hard not to give in and contact him but it's killing me seeing him with other girls. I've kissed a few boys in the mean time, tried to distract myself with friends and hobbies and all's fine when I'm busy and unaware of what he's doing but the second I hear or see anything about him being with someone else I struggle to deal with the situation. I want him to be happy etc but its really hurting me having to watch it happen in front of me.
How do you get over seeing your ex move on so fast? I've never done this before. Any help would be appreciated.