As a Buddhist how do I get over hatred towards my previous religion?
Think of the followers of your former religion as people who believe that the earth is flat. There's no point getting angry at them. It's ok to argue with them in social circles if you feel it might be useful to some and if you are confident that you can handle it. 

But if it is getting to the point that you start shouting at them or trash talking at them, you should stop and contemplate the Anatta nature of beings. You cannot make other beings think the way you want at will. Meditate on the wanting to control others' thoughts. Whenever anger arises in the mind, simply note it.