Difficulty is too high, I cannot finish main quest

I'm playing with my 2 kids on local co-op mode. We all have around level 90. We were doing some quests, but without a rush and few of them still are pending. We have recent eq that drops from monsters. Now we have a situation that we are completely blocked in progressing the game. Any place where we go, monsters kill us in 2-3 hits. When we fight against Elite mobs we are one shot characters. Not even saying about bosses. The gameplay in last days looks like we go out from the City, kill mobs for a minute and then we all 3 are dead. The same happens on easy and normal difficulty. I'm so frustrated of that, because I was sure I'm quite good in games, and now Im beaten by the game for kids.