I have a similar problem, I can get big diameters up the first 8" or so, a max of about 1.5" from there around the first bend to about 12" deep, then from there nothing bigger than about 3/4 inch diameter. Under that diameter I can go about 20" deep using 1/2 inch soft, thickwall silicone tubing with a moulded knob on the end.
At that 12" distance it seems to be more about a restriction than the bend, I tapered a nice soft 16" dong right down to about 1/2 inch on one end with a real gradual taper and it will go further in, but when I measure how far, it's just to the point where it's 3/4 inch dia is 12" along! If it was about getting around the bend, 3 to 4" of the dong has gone around so the rest should be able to follow.
Giving it a lot of time, lube and pushing over many sessions just doesn't make any difference, nothing over 3/4" goes past.
Anything under 3/4" goes in with little effort at all. I've pretty much come to accept thats how I'm built!
A couple of things I have found, some days are much easier than others, if it won't go easily give up and try another day.
You've got almost no nerves in there, so you won't feel pain until you're really rough and maybe doing damage, you feel almost all the pleasure from your asshole, so be carefull !
(You can confirm this for yourself by putting a hollow plug in your ass then then shove a dong in through it, you'll hardly feel a thing!)
Watching videos of men & women pushing 2" dia 18" long dongs (or a forearm) straight up into the belly, it seems their colon must be able to move in the body cavity (those things aren't doing an "S" bend) where mine does not seem to move at all, maybe yours doesn't either?
Maybe someone else is similar or has other suggestions?