Don't Panic (In large friendly gold letters)
Don't panic is probably the best, and most humorous advice I have ever come across for any of the kind of situations similar to this one or any other. If you aren't aware of the source for the reference, I highly recommend 'The Hitchhiker's Guide to the Universe'.
Freaking out and panicking is about the worst you can do in this kind of situation, because it makes you react instead of acting in the best way you know how to. Keep a cool head and you can usually deal with any situation fairly well, it may not come out looking graceful, but who needs to look good if you are effective? Anyways, easier said then done, but if you think it enough times it becomes second nature.
In either case it sounds like you handled the situation pretty well. Personally, I don't think your problem was that you didn't cast a circle, ultimately a circle isn't really necessary, it is more how you reacted once you felt the '**** hit the fan' and your fear based on your inexperience of the situation, which is pretty much par for the course, and something I am sure we are all familiar with. Comes with the territory when we are constantly jumping the boundary into the unknown.
You believed that your mistake was that you hadn't cast a circle and so you were unprotected against what you attracted, and so it was. The circle has no power, you are the power, you are what makes the circle effective or not, and so you can work without a circle if you believe you can. The circle is just sort of a handy tool that makes an obvious symbol for your mind to work with.
There are no mistakes except the situations you don't learn anything from. Sounds to me like you had a pretty successful experience, that you managed very well. Congratulations, don't worry too much about making mistakes, they are going to happen. Don't let mistakes keep you from exploring and learning. Keep up the good work bud.