I don't think, given your pattern of use its a comedown, I'm very familiar with DHC, its my opiate of choice, I get scripted quite a fair bit of it for a knee injury that never healed, well, it got operated on and that did nothing, so I have a longterm script, used it for years.
300mg/d for several months, will lead to withdrawal alright, I'm currently tapering down my use, so I can again go without and not induce withdrawal, and of course, use recreationally as enjoyably as I used to.
experiment, see how few, the absolute minimum daily you can go through without withdrawal, then split off a quarter of a tablet (I am assuming 30mg tabs, but go figure, adjust accordingly for dosage format) off each tab, per dose, and slowly step down that way, I found this to work well for reducing tolerance, although its slow.
And I reccomend something like clonidine or guanfacine, tizanidine, etc, these REALLY help alleviate the nastier effects of the withdrawal such as restless legs, as do benzos if you have access, just don't trade one addiction for another.
Quit while your ahead mate, a DHC addiction, whilst not banging up skag with bog water and rusty needles superglued to a ballpen barrel in a dark alley, can certainly make you sweat it out for a while, its taken me quite a long time to taper down, no puking and shitting, but the beginning especially was pretty rough.