Here is a pretty article from an advice site.
There are a few different exercises that you can utilize to slow your horse down when jumping courses. You are probably correct in your assessment that your mare is rushing because she is excited when on-course, but there also may be the aspect that she is a little nervous and anxious, or sometimes just wanting to get the whole process of jumping a course over with as quickly as possible. We can get really excited about going on a monster roller coaster at an amusement park, but there is also the element of anxiety and maybe a little nervousness involved too, so while your horse is probably enjoying jumping, she may get quick because there is also nervous energy as well. I'm sure when she gets quick, you're having to "get on her mouth" to try and slow her down, but that ends up giving her something to lean on and pull against (your hands) and that turns the nature of jumping into one of combat between horse and rider.
I would start back at the beginning by going to a low fence, like a wide crossrail. Just like any dog can swim when tossed in the water, any horse can jump when faced with a fence. However, when training the horse to jump, we are wanting to quality train, rather then "slop" train. Training for quality takes more time then just pointing and running at the jumps and hoping we get over the darn things in one piece. The lower, but more scopey (wider) jumps will cause your horse to have to round her back more and pull her knees up rather then getting flat like what happens when a horse gets quick and rushes the fences. When a horse rushes, they can't round their top line as easily because they are flattening while going too fast and they will tend to not tuck up their legs. They in fact will most often hollow their backs and sometimes "hang" their legs instead. Because they are usually involved in a "pulling contest" with their rider, they are also usually not paying attention completely to the task-at-hand (jumping), to their rider and to what they are being asked to do correctly, which is jumping the fences, so they get sloppy and start hitting fences too. If that happens, some horses just decide that they need to get over the fences as quickly as possible to get it over with because they associate jumping with hitting fences and getting hurt and this increases their anxiety level further. I would practice back and forth from the trot on the lower, wider fence and then practice it at canter. Make sure that you have ground poles so that your horse can focus or "sight" on a take-off point as she is approaching. As a horse gets almost to the jump, the jump goes into their "blind-spot" directly in front of them, so as they approach, they "memorize" where the ground pole and jump are and basically do a blind leap-of-faith. They hope the jump is still positioned where they have "memorized" it to be. The horse's eyes are set on the sides of their head, that's why a horse can see the jump from a distance, but as the horse gets into the last few strides and the jump is directly in front of them, they can't see it anymore. We see it fine because our eyes are on the front of our face, so here is where jumping becomes more of a teamwork effort; you will act as your horses "eyes" when you are almost to the jump. Sometimes you'll see horses that are bad "stoppers" on jumps with one rider, but then they jump confidently with another. Sometimes the reason for this is that they don't trust a rider to "help" them with getting to and over the jump safely. Set ground poles and give them something to "sight" on and memorize and they will feel more confident and pilot them to the jump with soft, steady confident hands and they will develop relaxation.
After you can school your single, lower, wider jump in a clam, relaxed way, add a second fence with about a two to three stride distance between and once again, approach the first fence from trot and canter to and over the second. It's like building blocks, as you are able to add on fences and not get into combat and a pulling contest with your horse, the whole motion of jumping a course will become one of relaxation and teamwork. You don't say what size horse you have and what her stride is like, but something else that I will sometimes do to draw the horses focus back to me, is set poles before and after jumps of various distances between, some are trot distances and some are canter. You can approach a fence with 3 canter stride/distance poles set up before the jump and as soon as the horse jumps the fence and goes on a few strides, have some poles set-up before the next jump with poles set up for a trot approach to the next fence. What you are trying to do is draw your mare's focus back to you where she is then instructed by you on what to do next in terms of approaching the next fence. If she continues on and gets quick on the trot poles at a canter, things are obviously not going to work out well. She will learn to "wait", not get quick and listen to what you are asking for instead of just charging ahead. Jumping is a teamwork effort, so your mare needs to learn to participate in jumping in that way with you. I don't ever "hang" on a horses mouth when on-course. I never stop a horse in front of a fence because I feel that is teaching them a bad habit and they may use it as an evasion later on. What I will sometimes do is smoothly approach, get as close to the jump as possible and if they start to get strong, I will smoothly circle-off and approach again. Only when my horse approaches on a light rein to the jump, will we jump.
If it gets into a pulling contest between me and the horse, then we are not ready to be out jumping a course of fences. I train by adding on one jump at a time when I know that the horse is relaxed, attentive and focused with what we are doing NOW. When we can do two jumps in a relaxed, quiet way, then I will add a third jump and train that. Then add a fourth, a fifth, etc. Do not move on to adding a jump until you can school the current jumps from a trot or canter approach, back to the trot between jumps, then canter on to the next, etc. You should be easily able to do changes of gait between jumps by "asking" your horse with a light pressure with your hands to "come-back" to you. Once your horse has done as you've asked, stop applying pressure or pull because we want to avoid getting into a pulling contest and we also want to reward the horse for complying by not pulling on their mouth when they have done as you've asked. The whole point is that you and your horse are communicating and your horse is relaxed and focused on what YOU are asking her to do. It doesn't matter if your horse can jump a course of eight or ten fences if its not done in a relaxed, focused and a teamwork type way.
What matters most is doing things in a relaxed, consistent and non-combative way. Be patient and take whatever time that it takes to train in a consistent, relaxed, focused way, with teamwork as your focal point. Use the "building-block" approach and don't add-on jumps until your horse shows you that she is calm, relaxed and focused on you and the task-at-hand. Keep thinking focus, relaxation, teamwork, patience and quality and your training will be successful with your mare.