Phoenix may disagree with me on this one. 
The two latex mattresses I have tried softened up quite a bit, and I am 6’ 180lbs. I walked on my second one, also firm, to soften it up initially, and that made a difference. It has continued to soften since then. Here is a link to my conversation with Phoenix:
https://forum.mattressunderground.com/t/second-latex-mattress-gone-soft
Hopefully you will love it soon, but if you are light weight you might speed up the process by gently walking on it, or walking on it on your knees, which is what they do to break in display models at my mattress store.
Lew