When I first started riding I was scared to let go with one hand and find the bottle. But when I began doing longer rides, it just became imperative to hydrate more and it wasn't always convenient to stop. Everyone has given you good advice: It just takes practice, kind of like clipless pedals. One day in the spring we were riding a long bike trail (on road bikes), so there were no cars to worry about and minimal bike traffic. I picked that day to practice, practice, practice, grabbing my water bottle. After that, it got easier. Good luck!