When ever I try to make homemade condensed milk it crystalizes, but when i first initially add the sugar to the heavy cream i stir til the sugar melts

The current issue of Fine Cooking has an article and recipe on making sweetened condensed milk - maybe this will be helpful to you:

http://www.finecooking.com/article/make-homemade-sweetened-condensed-milk

It's possible that you're over-reducing and there isn't enough liquid to hold the sugar, but if you're following the recipe that seems unlikely. This: http://www.southendformaggio.com/recipe-for-sweetened-condensed-milk/ says that stirring can induce crystallization, so try just stirring to combine and then leaving it be until it's done.