Whenever there is a problem, it's always my instinct to look at the rider first. Especially with a horse that is naturally more forward, it is very easy to lock your elbows and pull against the horse with your legs out in front of you which, instead of slowing them down, often has the opposite effect and you end up in a pulling match with the horse. Instead, sit deep and half-halt. When you ride, I'd try to see if you could have a knowledgeable friend or instructor watch and see what you are doing. Often times as riders we don't realize it, but we are doing things that contribute to- or cause- the problems.
Transitions as well as lateral work can be helpful with a forward horse. Put them to work and make them think. With some horses, they go fast when they aren't being challenged, but put them in a shoulder-in and you have a well-balanced horse who is on the aids. Good luck, and I hope this helps!