My shadow hit 50 the first week of January, he was my first toon from early access and I finished the class story at 47. I then did Ilum and Belsavis to cap him off. Survivability goes through the roof when you are fully spec'd tank and use a healer, in the case of the shadow stealth allows you to bypass any extraneous npcs along the way to your objective as well. Nadia is a glass cannon and Qyzen had zero gear on him at that point so for me, it was much easier to use the classic tank/heal combo and push through doing content that was 2-4 levels above mine (except trying Stark at 39, that was a mistake).
So Arvaliz, as I mentioned you need to find your playstyle and develop according to what's easiest for you. There is no singular correct path in this game and that is part in parcel of what makes it shine with respect to game mechanics.
you do have more survivability with a healer companion, no doubt about that. i leveled my shadow tank after i completed my guardian tank's story, so i was already familiar with the role. Nadia wasn't too squishy to me, but that was probably because i gave her my hand-me-downs. she was a lot easier/cheaper to gear up than Tharan and Qy were. also if you spam slow time, mobs shouldn't even be touching her. i also chose not to level with a healer so that i could learn how to tank properly and get used to my defensive skills. i know how much abuse my toon can take before things start getting hairy, and it got me used to popping my defensive cds. whenever i had a healer companion out, i'd get lazy and not pay as much attention to things like that.