Charlotte, there is two parts to this problem and they need to be considered separately.
1. Your fiance. You need to remove the girl you've been taking with from the equation and decide whether or not you'd stay with him even if you have no future with her. From what you write, I totally agree that you should end it. The engagement was too quick, he's too old, you've been fighting, his family hates you...and heck, you haven't even known him for a year. You should be still in the honeymoon phase!
On the topic of losing your friends. The general rule of thumb is that when you break up, he takes his original friends and you take your original friends. So, you should still have your school mates. And frankly, if your friends dumped you because you broke up with a boy - then they're really crap friends aren't they. Good friends will see your happiness as a priority.
Regarding him saying that he'll kill himself in response to you discussing leaving....that's called manipulation. He's guilting you into staying with him. Is this the kind of man you want to be with? Even if he does attempt suicide, it's likely to be a further push to guilt you into staying. You need to rise above his manipulation and do what's right for you.
2. The other girl. This may or may not work. You've never met her, so at present she's not much more than a fantasy. Just promise that you won't jump from one relationship straight into another. Hang out with her if you like and get to know her...take it step by step.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.