I can totally relate. In fact, my anxiety over leaving my girls to travel (ages 4 and 7) drove me to seek counseling so that I could get to the bottom of my fears and learn to cope with them, if not eradicate them completely. What I learned is that worry is nothing more than a habit and something that gives us the illusion of control. It makes us feel like we're actively doing something about a situation when we're actually doing nothing. What can help is setting up strategies or plans instead. For example, if you leave and something happens to your daughter, what will you do? Come up with a plan - you'll drive to the airport and get on the next plane out, you'll call in other family or friends to help til you can get there, etc. If she gets upset, what will you or your husband do to calm her? Make a plan for that too, or for anything else you are worried might go wrong. This helped me a lot because I was formulating solutions, not just stressing. And I was able to relax much easier. Also, someone gave me great perspective - what if something ever did happen to you? And what if your daughter had never been away from you? You are giving her valuable coping skills by leaving for a few days. She needs to see that she'll be just fine, even when mom is not around. And you need to see that too.
Another thing you can do when you are worrying is pick up a word search or crossword. These things are purely intellectual and so they take your brain out of the emotion. When the brain is free of that negative emotion, the body will follow and you'll be calm and able to think clearer.
Probably the thing that helped me most were energy exercises. Not everyone is into this, but it totally helped me with my anxiety. There is a technique called EFT, emotional freedom technique. I'm sure you could google it. It's basically a system using acupressure points in the body to release negative energy. When you are able to do that, it clears the mind and allows wisdom to come in. Your mind will shift toward clear thinking, such as the likelihood that everything will be fine. After all, something bad could happen to your daughter or to you at any moment, even at home. And you probably don't worry about that because you know the chances are very low. Being away doesn't really increase that likelihood at all, it just feels that way. Clear that negative energy so your mind can focus on this and other truths.
These things all helped me tremendously and I had a wonderful weekend away in mid-September. I hope this info helps so you can have a great trip too! You deserve it, you know : )