I'm in the same boat.. I feel bad as well but I once had a prof told me never to feel bad because it's a huge part of their job. I would suggest popping in to see them to freshen their memory of who you are and updating them on what you have been up to since you graduated. That same prof also told me she has written people letters who have been out of school 5+ years, just no more than 10.. so I think you are still good!