It sounds like your a bit frightened when you do lope, try to relax as much as you can. Think about when your asking for lope "encouraging" her into it. And when she does it will probably still be fast but keep riding and BREATHE out loud. Slow your legs your seat everything. Sometimes even singing a song slowly helps too. Practice these things and she should slow down. :) Good luck
I suggest lope her for a few strides, stop and back up a few strides. Repeat this 3 or 4 times. She should slow down because she'll be anticipating stopping and backing. Works for the trot, too. A local WP trainer suggeted it to me years ago and has helped me.
Firstly, if you're nervous on him, get a few lessons on her. There are no words to the value of a professional telling you and showing you the correct way to do things.
If you don't want to take lessons, there's a few things you can try.... I don't know your skill level (I'm guessing beginner, I appologize if I'm wrong) so I'll give you just a very basic idea to try. The big thing that would help you immensely is a soft mouth and that she gives to the bit. When you put pressure on her mouth, she should give to it. If she's pulling against you, that should be your first mission. From there, it can be just a give and take. Hold her for a few seconds until she slows down a notch, then release. She'll speed back up, but it takes time. Lots of repetition. Don't worry too much about stopping, backing up, and repeating until you have a bit more control over the lope. That will help more when you're looking for consistancy and needing her just a little slower. Depending on your horses temperment, that could get her hot and bothered. If she's lazy, though, it could help.
I won't discribe more technical things to try because it probably won't make much sense. Things that like need to be shown, not read. I think a few lessons are your best option though.
Definitely take jumperwithfire's advice getting relaxed and comfortable. It will help you immensely.
Did you know that humming works well so hum something boring. It works on horses as well as babies. I've often hummed only two notes, higher and lower, kinda like the ding dong of a door bell, at the speed I want the horse to take.
The singing idea is useful for another idea:
If you sing a song that has 3 beats to it, it will help you kind of maternalise the speed you want to go.
You've got the 1, 2, 3, thought down. If you don't want to sing, simply think the numbers slowly to the spead you want to go. It will take more than thinking to do it, though. Leg work to keep your seat the speed you want to go is what really counts.
Keep in mind that getting a horse into a slower lope involves a lot of things.....you must be relaxed and balanced, your horse needs to be relaxed and balanced, your horse needs to be collected, and your horse needs to be physically able to lope at a slow pace. All the suggestions above are a great start to slow it down a notch or two, but you may need the assistance of a professional trainer to really get you and your horse to a slow, collected lope.
You might try getting your horse tired of racing by galloping him in the largest arena you have available and keep pushing him when he WANTS to slow down. If you have several sessions of this it won't be as much fun to race.