Well, in that case, I would suggest giving it a try. Like I've said before, it's only slightly larger around than a tampon, but it's also not such a compressible material - you can only squish it so much to make it so small. I'm not sure if your endometriosis pain comes in waves or not, but you could try transitioning slowly until you get better at inserting it. Try it only on a good day when the pain isn't too great.
I noticed that when I first started, putting it in was most painful and yanking it out was easy peasy. Now, I'm an expert at putting it in, but sometimes I yank it out too quickly or don't put enough effort into pulling it out carefully and it's actually more uncomfortable on the way out than the way in. I think the bottom line is how carefully you treat it when you have to insert or remove it. If you don't traumatize yourself by switching cold turkey, you'll give yourself a chance to learn how to do it better at your own pace.
The other nice thing is that if it IS the dryness that hurts with the tampons, you won't have that problem at all! And if it isn't the dryness, then you just don't have to take it out as often as a tampon because it doesn't absorb all your natural juices and it isn't bleached or full of other chemicals. It's still a risk for TSS, but not nearly as high as with tampons. On my slow days near the end of a period, I've been able to leave it in all day, only emptying it when I woke up and again before bed.
Not dead yet.
If you go ahead and take the plunge, keep us updated! I'll be glad to coach you through the learning stages.