My mistake is keeping my head down and driving. Then I get caught in the guillotine.
That's what not to do. Even in wrestling, where you don't have to worry about the guillotine, if you keep your head down and drive forward, it's like pushing a car. You'll tire yourself out and get nowhere.
To deal with a sprawl, there are a lot of things you can do, none of which you can learn very well by reading a forum, so maybe take some of the suggestions and practice them, and ask your instructor about them.
First of all, you need an angle to finish the takedown. You can post a leg on the outside, the same side you want your head to be on (not sticking out, so as to avoid the guillotine), cut the corner and finish the double as if you re-shot on him from your knees, continuing to finish in a circular motion while driving across his body (on the side, as opposed to straight into him). You could also pick a leg and switch off to a single. For that, you would want to circle outside towards the leg you take, tripod, and then try to come up with it, and get your head up. If the guy is keeping your head down, he's stopping your takedown no matter what. Don't come around too far towards him on the single, or he will overhook your arm and either stop your takedown completely, or try for a head/arm submission. Another option would be a sit-out, depending on where his arms are. I find this best when you can control one of his arms, and then sit out to that side.
Hope you find this useful.
**edit....To improve your double leg, drill and practice. You can never do this enough. Elite level wrestlers will still drill this move hundreds of times. It takes a lot of practice to get it good. Also try setting it up with some hand fighting and so forth. A lot of non-wrestlers tend to neglect this it seems.
