When I hit 50 on my healing sage last March, I was have survivability problems with my sage healer in HM's and over 50 PvP, I wasn't able to solo the H2+ on Ilum either.
But, my problems in HM's was caused by tanks who couldn't hold aggro & dps that wouldn't peel. My problem in PvP was that I was just bad. As for the Ilum H2 - again I was just bad.
As I earned my daily comms & mods from missions rewards I prioritized End over Willpower just like you have. And my problem got better. It wasn't that the extra End gave me the HP I needed to stay alive - it was that I learned how to play. By the time I was in Tionese level gear I had HP like a Shadow tank. But I was able to heal HM's just fine with that little Willpower.
Come to think of it, a high endurance will actually hurt your sustainability if you are a healer. Healers rely on Noble Sacrifice for sustaining their Force points.
This is absolutely true... As I gathered up my Columi pieces I started dumping End. Healing became very comfortable, and by the time my Columi set was complete ~ I had the same amount of HP, with a massive boost in Willpower. I had already learned how to be careful with noble sac at that level of HP. The thing is, as you gear, you will get more HP, and Noble sac is always becoming less & less effective.
It's tough to heal without using things like cleanse, interrupt, clickable relics, stims, force armor, Rejuvenate, and the Aoe heal. But a High End build will require all of that from you, or you will drop a whole lot of players in your early HM's. With a Low End build you can spam the AoE heal & noble sac your way through most of the HM bosses.
All that being said, most the sage healing prowess comes from the base healing done by our abilites ~ and the bonus healing from stacking willpower/power is sort of just "gravy" healing.