DXM generally doesn't mix well with other drugs. Tricyclics, SSRIs and stims almost always cause dysphoria (at least) when mixed with DXM, and MAOI/DXM can be fatal.
That being said, the most likely reason that your trip is so long and unpleasant is the addition of Seroquel, because quetiapine is a dopamine, seratonin, and adrenergic antagonist, all three of which present certain complications when mixed with DXM. However, add to this the anticholinergic properties and I can easily understand why you feel so bad.
That being said, the doses you described are fairly benign, assuming you are not enzyme deficient with regard to DXM/DXO conversion (which I assume you aren't, given your past DXM history), so the risk of any substantial harm is pretty low. You could conceivably experience seratonin syndrome from this combination, though in the doses cited I would imagine it would be a low grade manifestation of aid condition. Seratonin storm can be fatal, but it's rare (I've had it more than once, went to the ER twice with it and found out both times that my fears were wildly disproportionate to the actual risk, though the dysphoria was quite intense), and while it must be taken seriously, I sometimes feel that the actual risk of death from SS gets overstated in some of the threads dealing with it. In the spirit of harm prevention, I would recommend that you immediately acquire a sitter at the very minimum, and if you are really battling with yourself, do not hesitate to check into the ER. Remember, there is no legal problem that remotely compares with serious injury or death, not to mention that what you have taken is most likely legal to begin with.
I have also experienced bad DXM/some other drug combinations, including seroquel, so I do understand exactly how intimidating it can be. Indeed, I have been downright terrified for hours on end in such situations. I understand, and its no joke.
That being said, the most important thing you need to do is acquire your wits and apply your mental discipline to AVOID PANIC and APPLY LOGIC. Panic can lead to bad decisions, such as adding a benzo to take the edge off the fear -- bad idea -- point of fact, adding any drug right now is a bad idea, and will not make you feel better. What you need now is the most rapid and safe method of detoxifying, not increasing the amount of drugs in your system.
Remember, at the end of the day there is no shame whatsoever in rushing to the ER, and if you find yourself unable to control your fears, or if the symptoms of dangerous conditions increase, by all means go in.
I don't want to minimize the importance of this, but I have been through this combo with a much, much higher dose of both and survived, though it was quite scary for a whole day, and it took another couple days to "feel normal".
If you are already feeling better, just remember that in all likelihood you're fine, try to chill physically and focus on something mentally challenging, like solving a Rubik's cube (works for me in tough psychedelic situations) to refocus your mind on something other than how negative things are. Please keep us updated so we know you're okay, and remember to keep your DXM days separate from your other drug days!
Hope this helped!
ADDENDUM: As for the risk of permanent damage to your brain from this - very very unlikely. DXM has a way of really biting when it goes bad, and that feeling of "never coming back" is common in any negative psychedelic experience. So as far as that aspect goes, rest assured that you will absolutely feel normal again, most likely within the next day or two, and you won't even feel as bad as you have for the rest of the experience. It will let up gradually, and in fact could even become pleasant once you realize that you are out of harm's way. And no, there are no lesions or holes in your brain from this!
. I am curious if I had mild serotonin syndrome, although wouldn't the combination of a seratonin antagonist make it harder develop? I read on wiki that 5HT2A agonism reduces neurotoxicity, would an antagonist have the opposite effect? Of course the seroquel and promethazine especially are potent anticholinergics and inhibition of choline receptors is said to reduce neurotoxicity.