Crushing on my friends boyfriend!?

Now before you think I’m an awful friend, let me give some details on my situation. So there’s this guy, let’s call him ‘Leo’. Me and Leo were best friends in primary school and I secretly had a huge crush on him, but then he moved schools and we lost contact. A few years later we ended up at the same high school. We never really rekindled and I no longer had feelings for him. Now I have this friend, let’s call her ‘Milly’. We’re not that close, we just happen to hang out with the same group of friends. Milly and Leo started dating a few months ago, and at first I was happy for them, because even though me and Leo weren’t friends any more I knew he was a really sweet guy. But then Leo started to spend heaps of time with Milly and our ‘group’ and after getting to know him again, all those old feeling are coming back! I feel really bad because, even though Milly and me aren’t close, shes never been anything but nice to me. What should I do?