Sounds like it's time for you to move on, then.
DracoSuave is spot on. RPGs have a GM for a reason. No amount of rules will ever be able to replace a GM so it's not a reasonable goal to have a rule for everything.
Sure, but I don't think it is particularly helpful, in a rules thread, to say, "The rules don't matter - let the DM decide." Even when I agree with that sentiment, I don't find it particularly appropriate here.
Especially since the rules do, in fact, address this. PHB pg 268 lists the following order:
-Ongoing damage occurs.
-Regeneration occurs.
-Any other 'start of turn effects' occur.
-Any effects that 'end at start of turn' occur.
My group has tended to play that the acting player gets to choose the order when things might process at the same time. (Say, if you start in the aura of two swarms, one of whom has an attack that would push you out of the others aura.) But those are pretty rare cases.
And, of course, it should go without saying that a DM is perfectly free to change this or rule otherwise, as best fits their game... but for those looking for some guidance in the rules, the above seems to be what is available.