As the old saying goes, 'If you can't take the heat in the kitchen, don't cook'
Anyone who gets on a horse can expect to fall off sooner or later. Get use to the idea. Do it often and, like most things, the more you do it the better you get at it.
Of course you are nervous, you are a total novice, have gone out and bought a horse when you have little experience and probably no help when you ride.
You need to be riding under instruction. You didn't say if the horse did anything wrong or if you just lost your balance.
The only way to get over fear is to face it head on. Get an instructor who will not allow you to get panicky, climb back on and kick on.
This above is tough - so is horse riding! You will not get sympathy from me, it isn't in my book!
. I rode naughty ponies and had many falls and injuries. When I arrived home, my mother would just laugh and if I moaned she told me to stop. The sympathy was, "worse to come before you die" or "worse things happen at sea, you don't drown riding horses" so you can see where it came from.