When you opponent has his legs crossed and is sitting low on your body, do the following:

Extend one of your legs fully and drive it to the ground seperating his two legs.  You may have to rotate on your hips to wedge it in.

Push off his hips and shrimp your legs out from between his legs.

Related posts:

1. Step Over Escape from Full Mount
2. Triangle From Full Mount
3. Tips for Turtle Guard