what's the difference between 羡慕 and 妒忌？ Can someone tell me the difference between these to words. Are they both regularly used?

They seem to both indicate jealousy. I've seen 羡慕 on different occasions and I'm used to that one, but I've come across 妒忌 a couple of times lately and wanted to know if they were interchangeable.
羡慕 is a good kind of envy. 

"You have a happy marriage. I'm really envy at you and I hope you can be happy forever" This is 羡慕.

妒忌 is a bad kind of jealousy. 

"You have a happy marriage. You don't deserve it! I should be happier than you!" This is 妒忌.