I agree with Cali and Ionic as well. I was having a TON of difficulty staying vertical. Check your shoes and pedals. I found I had chipped the cleat on the bottom of one of my shoes so I replaced that. Still fell. So I changed pedals to a Ritchie model and have had no problems since (at least that I can blame on going clipless )

Hang in there! I was at my wits end with mountain biking but am back in the swing of things. I also found after a couple bad crashes that nerves were playing a big part in it so I started road biking more. Much easier for me! It has been a great confidence booster as well as a great endurance training tool for me.

Happy riding!