To me, the Assumption is easy to believe IF you believe that Mary was without sin. Others were close to God and were taken up without dying (Enoch, Elijah and of course, Jesus). I can buy into the idea that if you didn't sin, or if you received a special cleansing from God, that you don't need to experience death. Getting to the "without sin" part with Mary has been the more challenging effort for me.
The difference between Jesus and the others you named is that Jesus ascended by His own power and authority. The others were taken up into heaven. Did you see the link I gave no_outcome above?
I agree there. Part of the problem is that it is a very theologically complex doctrine. I can see Mary as being a great and Blessed Mother of God. I can accept readily that she made no habit of sin, or that God may have strengthened her spiritually in view of the Incarnation, so that she commited no sins. But the doctrine of Original Sin is itself complex, so I guess it is where I get confused.
Most protestant theologians explain that Jesus can be free of Adamic Sin since that is transmitted through the father's spiritual genetics. However, they might also be wrong, since the kingship of Israel of the Davidic line is also supposedly reckoned by fatherhood also, and Mary of course is female.
At any rate, If I can come from the Regular Baptist protrayal of the Mass as blasphemous idolatry to the correct understanding of worshipping Christ via the visible elements of His Body and Blood, I have come a long way so far.