Help with combat - What changed so much?

I played this game twice as an elf mage. Both times were really quite easy. I never lost much time researching potions and only crafted best weapons and armor for my mage only, companions only got loot. Spamming ranged attacks was quite easy.

But now I'm playing as a rogue dwarf (dual wield dagger) and it's much harder than I remember. I got my ass kicked by Crestwood's Dragon a lot of times before I could beat it, and it's supposed to be the easiest. My party initially was me, Iron Bull, Blackwall and Vivienne. But Iron Bull is just such worthless and kept dying so I switched him for Solas, but it was still hard. Vivienne died a lot while I was out of potions and I had to keep ressurecting her. I made sure none of them had electrical staves. 
Also, playing as melee is fucking weird. A lot of times my rogue just hit air when I tell her to use deathblow or twin fangs. It's really hard to position your rogue for flanking, especially against dragons. 
I was able to defeat Hinterlands' dragon with Iron Bull and with only 8 healing potions, while I had 12 against Northern Hunter. 

Mages usually don't stay on the way I tell them to stay, and often I see them charging on melee. They also seem to use barrier on the most stupid times, and I've seen them sometimes not even auto-attack, just waiting for some skill to cd. Blackwall is also having serious issues in holding aggro when fighting multiple enemies, because I'm getting attacked all the time (and so are the mages), and when I switch control to him I see "Challenge" and "War Cry" are already on cooldown. 
I'm also having serious issues when I fight groups of red templars (on one fight, my group used over 10 potions)

I'm level 13, dunno what I'm doing wrong.