Hi Eliza I was going to reply the other day and somehow I got distracted... muffin problem was exactly what brought me to this forum!!
I am not sure your problem is the texture itself, but if not, the texture is right but not growing into big fluffy muffins, read the first thread I started here
... I tried the technique from Shunka (near the bottom), as well as making sure to use a freshly opened pouch of baking powder and the things got much better!!