How can I keep myself from falling again?

Then slowly back away. Don't be so close of friends and stop going to parties or places he invites you to. He can be an acquaintance, but if you're always around him, then the feelings may spark up again. So I guess it depends on whether you want to get back together or completely move on. Because if you want to completely move on, you need to remove all the constant communication or other reminders out of your life or you will never be able to move on.