I have a lazy pleasure horse too. The gray in the picture to the left. But even though she is lazy I had to work on her for like a year to get her to slow down her lope. You have probably already tried all of this but I'll tell you just in case you havent. I did a ton of small circles, basically as small as she could possibly do. I also used draw reins and got her really collected up. My sister always commented that she looked like a peacock, but I wasn't worried about her headset at the time. Eventually she realized that it is easier to just go slower than to have to be so collected. One thing I didn't do was slam stops because that just got her reiled up and she wanted to go faster. Now that I got her going slower she drops her head down and does a real nice lope. Oh, she also is a big horse and can not go as slow as some of those other ones, but I don't think that really matters. As long as the horse does a true lope and none of that 4 beat trot lope unatural ugly stuff they don't have to go as slow as the other ones.
One time I was at a show and there was a horse doing the ugliest 4-beat I haev ever seen. The judge asked the class to pick up and move with forward motion, but she didnt. So after the class the judge actually went up to her and said "your horse moves like crape"
I thought it was hilarious