it's great you want to start riding again! you should just take it really slow, if you don't feel like riding yet, just be around the horses and barn and other riders. i know falling is scary, but it's usually not as bad as it seems at first. i've only fallen once in like 6 years, but it wasn't too bad i was just really sore after that. (i fell in to a jump and then the horse almost stepped on me.)....lol i'm kinda getting off topic here.
maybe you could do some balance exersizes on the horses (like vaulting or something) to make you feel more secure on the horses. but it's great you're getting back in to this! i wish i could ride again