I'm 30 so this isn't one of them crazy plots to get him back (unless its possible haha)
I was with my ex for 5 years, we lived together for 1 year as a couple.
Then I got scared at the fact I was his first relationship and I split up with him, (In a sort of set him free act).
We stayed best of friends, because we still loved each other and lived together, with other friends, a further 3 years.
He began seeing other girls and I started seeing other lads. But nothing serious.
Then my ex and I started having a little "fling" last year, and I thought we may get back together.
I never said anything, and then he started seeing a girl. They have been together 6 months and they have already started talking about getting a place together. I am heart broken and I really thought that in the end like most tacky films we would end up back together, once we were ready.

What do I do? Is there anything I can do?

No you can not win anybody back. There is no prize.

Ex is an ex for a reason. Time for you to move on.

There is not much you can do now, he has a girlfriend.

Leave him alone to enjoy his new girl. Sorry it didn't work like in the movies, but real life sucks sometimes.

One thing you can do is be honest with your feelings about him (and to yourself) ask yourself these questions: A) Do I just want him back because he has found a new love and they seem serious? B) Would I want him back if he was single? C) Is he the right guy for me, long-term? Once you discover the answer to these questions, it may help you better decide for yourself... what you should do. If you answer "no" to either one of these questions, then you should stay in the background and remain as friends, in my opinion. Good Luck.