Should healing be this difficult while leveling?

[Renryuu]

Heya, so currently I'm leveling Whm which is the last healer for me to level to 70 and I'm feeling like I may be missing something. With Sch I had it when I leveled Smn to 70 so I don't have experience on it and with Ast, it was my first healer to 70 and at the time I thought the problem was just me having not been a healer for such a long time and having to relearn as well as learning Ast. Now I'm leveling Whm which was my main healer back in ARR and once again I'm feeling overwhelmed like I was with Ast.

I just did Sirensong and had a War tank who used Defiance on pulls but swapped to Deliverance afterwards for the rest of the pull and I was having to expend all my mp just to keep him alive on trash packs. The only time I got to dps on trash packs was on the very first pull of the dungeon and the last part with the 2 dogs and the one single mob. The one time I tried to dps outside of those instances, the tank dropped like a rock so after that I had to go nearly empty on every pull to keep him alive. Boss fights were fine and things felt normal, but when it came to the trash, my goodness I felt like I was horribly missing something.

I know at 70 I still struggle with trying to heal, dps and use cards with Ast, but with Sch I do feel like I'm able to heal and dps fairly competently, but now with Whm and I'm sitting here wondering what I'm missing that makes Whm the easy mode that I see people say it is.