Jessica,
I'm sorry to get back to you so late. I was trying to post to you last night, but my internet connection was terminated, and I was too tired to bother with getting back on. Anyway.... here I am now.
Part of your problem was in the application. You are not supposed to just slather it over your head. You are supposed to take your hair section by section and work the henna from roots to ends. This is time consuming. Had you used a colored henna, you would have uneven color since the henna most likely was not able to penetrate every strand.
When I removed my henna, I got under the shower and let the spray start to rinse it out. I used my fingers a bit to loosen it up. I then applied conditioner and let that sit a bit before rinsing. I applied more conditioner and used a comb to comb out any remaining henna. The combing was not at all difficult, slid right through with the conditioner on there. I just rinse the comb after every stroke to remove the henna from the comb. This worked very well. I then rinsed again, and conditioned again, before a final rinsing. Some shampoo, but I read somewhere it was better not to, so that was the way I did it.
I think not really knowing how to apply the henna made it the most difficult for you. You applied it upside down, so your hair sort of froze that way. I applied right side up, then put the length of my hair up on my head and covered. It never fully dried out. I could take the length down just fine; it wasn't stiff. Hopefully, with a few conditioning treatments you'll be loving your hair again. Other than that... Jade21's suggestion for you to wait until you were happy with your hair's health to apply, may also be a factor.
Hoping your hair is in a better state tonight!

-Fox
17/31/35+ Type 2ABMii (3B underneath layer)
Somewhat fine, slightly wavy (with curls on the underneath layer), light brown with gold and red
highlights.
Products Used:
Aubrey Organics S&C
Vinegar/herbal rinses
Oils: Jojoba, Sweet Almond, Coconut & essential oils
MP Popular Mix Brush
Wood and Horn combs
Vitamins/Minerals/Herbal supplements