I know your event probably already happened, but I'll tell you what I try to tell myself. I say, let this be a test to see if I can get through the nightmare of not sleeping at all before a big event and live to tell about it. I recently had to work a big exciting job and couldnt sleep a single minute the whole night before. I layed in bed for 9 hours not sleeping and then worked this big event all day. it was painful, but after the first couple of hours i apparently became so tried that I was making everyone laugh and have fun. I was too tired to worry, and it was actually easier to be social. i was even joking with famous people. who knew?
Social anxiety is tricky. You're going to have to accept that you will be uncomfortable and then forgive yourself for it. When you are social, your body fills with adrenaline and the sensation can be scary. It is a chemical that is meant for high stress fight-or-flight scenarios, so when you set foot in a big room full of people (including ex's) and then your body rushes and your heart pounds, it is really a chemical reaction that is TOTALLY normal. When you feel it, accept it. Know that your body is doing exactly what its supposed to do, and in ten minutes the chemical will slow and stop and you'll feel normal again.
A good trick is to give yourself an easy activity to do for those first five or ten minutes while your body is adjusting. enter the room and then excuse yourself to make a phone call. enter the room and go straight to the bathroom. then come back out when you feel more comfortable.
Also, a great thing is to remember that EVERYONE is much more preoccupied with themselves than you. We are all busy worrying about what everyone else thinks of us. Do some people-watching. Watch how they walk by windows and look at their own reflections. It will make you worry less about what they think

Maybe if you can tackle those anxieties (and i know Aspergers makes it tough) you will feel less nervous about sleeping the night before. Its really hard when you know your emotional and mental stability is dependent on your sleep. I'm the same way.