Oh yes, I feel your pain! I one-shotted Rom and strolled into Yahar'gul thinking I was going to steamroll the rest of the game. I quickly got my arse handed to me. There is a definite ramp up in difficulty after Rom. Here are a few things that helped in Yahar'gul:
- Level up. I was around level 50ish when I first got to Yahar'gul. After levelling up 10/15 levels, having more stamina and health is always a good thing.
- Level up your weapon. Ideally, get your weapon up to at least level 8
- Take out the bell ringing maidens. If I recall correctly, there are four of them and things are more manageable when they are gone. YouTube walkthroughs can help with their location, and EpicNameBro's videos are very good.
- Run! Do suicide runs from the lamp at the start of the area until you've found the second lamp. The area initially feels a bit confusing, but it's a pretty short distance from the first to the second lamp.
My route was to do Yahar'gul, then Cainhurst Castle, The Nightmare Frontier, Upper Cathedral Ward, The Orphanage, then the Nightmare of Mensis. I haven't touched the DLC yet, but I understand it's best to tackle it just before the final boss.