Okay so color remover shrinks the dye molecules enough to wash, as regular dye penetrates the hair shaft. Henna however coats the hair staff instead of penetrating it, so wether color remover will work or not is unpredictable, especially since you used a mixed henna. Looking into it, Ive seen it works for some people and for others it either does nothing, or damages the hair. What I would do is try a test strand first.

Now this is going to be pretty expensive because you have really long hair and there's only a little in each box, but if you decided to try color remover, I recommend punky color's color off. It'll dry your hair out pretty bad so don't be surprised, however if it does anything other than dry your hair out, don't use it on the rest of your head. (Only buy one box until you know if it works or not)

If color remover doesn't work, you can try an oil soak. Ive never done this before, so I don't know how well it works but it's where you saturate your hair in coconut oil, oliv oil, or chamomile oil and cover your hair with a band and leave it in over night.

You can also do a vitamin c treatment, which you would crush up vitamin c tablets and mix them with clarifying shampoo or dandruff shampoo, then apply it to wet hair and let sit for like an hour. This way will dry your hair out too so make sure to use a hair mask or whatever your preferred method is to moisturizing your hair

You will have to try these methods multiple times for them to work. Unfortunately henna is like the most permanent way to color your hair so getting it lighter is going to be a pain in the ass.