Cowgirl position

My boyfriend really loves cowgirl And I like showing off for him. However, I have a hard time having him stay in. I get like a slip and slide down there 🌊) and my hips hurt after a bit. I am fairly inexperienced with any of it., but I am pretty flexible (legs over shoulders? No problem!) I am looking for tips to help me improve and feel more confident in this position. Also we are very long distance. Are there exercises to stretch and strengthen the hip flexor muscles to practice while I wait to see him again?