Welcome to Knitting Help!
I don't know which method you are knitting -English with working yarn in righty hand or continental with working yarn in left hand- but trouble purling isn't uncommon when you're learning.
Bring the yarn to the front between the needles Then you plut the right needle in the stitch from right to left and wrap the working yarn from right to left over the top and back and pull through the stitch. Don't think if it as down. It's more right to left regardless of how you hold the needle. It's hard to explain, but maybe a different explanation will help. I put the main page so you could choose your knitting method.
It will take practice to be comfortable with any knew technique, but you'll get it..