Riding Help?

Sarah
Alright ladies I need y'all's advice... My boyfriend wants me to ride him. I've only ever done it twice and both times were soooo difficult! I'm 4'10 so I feel like my biggest issue is being able to anchor my knees on either side of his hips. When I practice alone on a toy I'm fine, so I have a feeling it's definitely something to do with anchoring myself. 
Any advice on how to remedy this? (P.S. Y'all are the bomb)