of course your not a horrible person!!! we all do this from time to time - but one of the worse things we can ever do is compare ourselves to someone else - you have to remember somebody will always be prettier, thiner, richer, younger, happier etc.. than you.
To be honest it may look like she has the perfect life but, of course she doesnt! - A girl I know recently told me that she was envious of my life - had a lovely OH (ha ha - yeah lovely but we fight like cats and dogs - behind closed doors ), a good job (It sounds way better than it is), way more money than her (rolling on the floor laughing)... point being.. it looks that way.. you have no idea what really goes on in their lives, regardless of how good your friendship is - we often tend to tell most of the sucess in our lives and leave out the bad for fear of embarrassment etc..
It's funny, they say that if everyone took their problems to a table you would take your own back with you. Think thats true. She could be secretly jelous of you - you just don't know.
My advice is to concentrate on what you love about your life - I found myself being jelous of a colleague a few months ago (single, no children, "rich daddy", weekends spent lying in, shopping (everything she sports is designer) and goes to spa at least once a month, no responsibilities whatsoever (still lives at home, has a car but her daddy bought it)
But then I stopped, thought about it (we're the same age) and looked at all the plus' in my life - I will be well settled in my house (more to point mortgage repayments) long before she starts thinking about buying or building, will have my children "well on" before (by the looks of things) she even starts having them, etc...
It may be great to go home from work and have nothing to do other than you hair but, imo, its even more fantastic to go home to my beautiful daughter, my Oh, my own house, in my own car... (am I making any sense?)
Thats not to put down her life - we just have different priorities and it suits us each perfectly!!.