I'm sorry you're so down hun! I'm totally not a dr and I only know my own experience, but could your period just be so crazy because you haven't had one in so long? I would think there would be a lot of build up and it needs to get rid of all that stuff - this could be a very positive thing, your cycle getting back to normal. Either way, you could definitely talk to your doc (RE?) about it. A 21 day period would suck no matter what!
As for the constipation - I find that lots of veggies and regular exercise keeps me very regular.
I've also heard that fibroids can cause a lot of "clotting" during your periods. I'm just throwing that one out there - I really don't know much else about it.
And endo does not automatically lead to a hysterectomy. There are quite a few of us who can attest to that
