Double crease under one side of my buttocks. What is the best solution to help get rid of this crease? (photos)

This situation can be difficult to resolve. There are two options that I would consider. One is a very superficial Liposuction which would have to be performed by a highly skilled Doctor, otherwise it might lead to a fold that isn't very appealing. If this doesn't work, the other option would be to do an excision of the skin between the two folds but this could create a scar there.

Your lower crease appears normal but your upper crease looks unusual and worse on the left side of the photo. I think this could be improved with some fat grafting. Discuss with a plastic surgeon who does a lot of fat grafting and you like their results. Good luck.

Hello dear!
Thanks for the pictures and provided information as well.
Based on your photos, you are a good candidate for fat transfer to fix your problem, and I think you'll get very good results. 
l recommend you to make an in-person consultation with a board certified plastic surgeon for a proper answer and to talk about your goals and anything you need to know.
Good luck :)