Defenders are a force multiplier. The problem, I think, is that you're playing a character whose primary that rely fairly heavily on "anchor debuffs" and a secondary whose secondary effects aren't all that noticeable. In a protracted battle against a few tough targets, you'll really shine. For the remainder of content that teams typically bulldoze through, not so much. As for "folding like a tower of cards", all I can say is to take a defensive armor toggle from an APP/PPP and, when able, slot IO sets that grant additional defense. Since defenders also get the best bang for their buck from the leadership pool, I'd take some powers from there as well. Again, time and resources permitting, getting your hands on 5x luck of the gambler global recharge IOs will also help you out a bunch!