The best way to slow a lope is lope, lope and lope some more. You don't say how old she is. If she is young she may still be trying to figure out how to balance with a rider, if she is older maybe she never learned how to lope because she has not done enough loping. I like to find a long sandy trail and lope usually they will start out fast but will slow down if you go for long enough, usually within 5 or 10 minutes. She will learn to slow down on her own, she will come to realize she does not know if she is going for 5 minutes or 5 miles and will learn to conserve herself. Once she learns on her own how to balance and slow down then you can teach her to collect and work off her hind end. If you are in an arena lope using the same idea, but try to just be a passenger and don't over steer her, try to not touch the reins at all so she learns to be responsible for her own feet. She may duck, dodge and get stuck in corners but keep her moving by applying pressure as needed, otherwise leave her alone. Vertical collection comes after they learn to balance.