piper wrote:
Thanks! I think the biggest problem is that that side of my thumb is quite sensitive skin-wise, specially around the nail.

So basically, If I got it right, I should always be pressing both A and D strings with my thumb?
Thumb position is very uncomfortable at first, but if you do it the right way, the discomfort will soon pass. You will gradually build up a callous near the first joint and then another by the nail. I agree that the nail part is the most painful. I started working 5 to 10 minutes at a time and then gradually increased as my callouses developed. When playing the cello, it is best to avoid changing the standard way of doing something just because it is uncomfortable at first, because most everything about playing the cello is uncomfortable at first. Also, New Skin liquid bandage may be your thumb's best friend while you're developing callouses. Smile