I think sometimes the fear that the WS might relapse or that we may have been fooled again is what fuels this notion that we have to be "tough". Problem is, there's no amount of toughness which can truly insulate us from being hurt again. Some WS's do relapse and some do fool their BS. We hear about it all the time. It's awful. But that's not all the time and it's not even most of the time.
Hypervigilance can't protect us, not if we're determined to leap back into intimate relationships again. In some ways, it's a product of brain function during trauma. The amygdala seem to be stuck somehow, triggering a biochemical stress response which can feel exhausting after awhile. When we really start digging in to our Fear of Abandonment though, it's not logical. In The Body Keeps Score by Bessel van der Kolk, the author points out that the connection between the prefrontal cortex (judgement) and the amygdala (fight, flight, freeze) is kind of tenuous. You can't really talk yourself out of trauma. That doesn't mean we can't grow into a better understanding of where those abandonment fears are coming from. Once you really understand that it's a false reflex at this point, sort of a secret stowaway from infancy, we can use that knowledge to start investing in our own resilience.
I won't say that my fWH can't hurt me anymore. That's not true. If I found out that I had judged poorly and that he had fooled me, I'd be really sad... but not destroyed. And that's the difference. I really was destroyed in this. Bug meets windshield. Everything I knew, everything I believed in. somehow got tied up into this trauma. Everything. Love, commitment, identity, family, faith, fate, luck, you name it. This broke me. I've had to rebuild from scratch in many respects. That process happened. It can't unhappen. I'm a different, stronger person than I was when this all went down.
In some ways, our fear of getting hurt again is a false fear, and not because it can't happen again or because it won't hurt, but because we are STRONGER for having had this experience already. It tempers us like steel and we're not even aware of how much we've grown. On dday, something is severed, leaving us scrambling for a fingernail hold in a world that suddenly doesn't make sense anymore. We have to right that world on our own, and having done it, it can't be undone. Now that we KNOW we are enough and that we can handle whatever's thrown at us, we're fearing something that can't ever happen to us again in the same way it happened before. WE grow and change, and yeah.. it's painful, but we become strong. It's like relationship bulletproofing. They can wing us, but we'll never get got through the heart again. 
R requires vulnerability. It requires risk. There's no way to insulate ourselves emotionally from that because what we're talking about is essentially an emotional relationship. If you read a copy of What Makes Love Last by John Gottman, you'll see that trust is a multifaceted concept. What I found is that I could allow vulnerability as long as I had faith in my ability to recover if I was proved wrong in my trust. IOW, the one I most needed to trust was ME. I needed to have faith in my own resilience, in my back-up plan, in my growth. The infidelity had shifted my dependence from "us" to "me". Instead of enmeshment, I could seek out sharing. Instead of "two halves making a whole", "two wholes working in harmony". Instead of reliance, enjoyment.
Anyway, long post made shorter, boundaries are important, but the purpose of them is to create a healthy amount of separation from others, to define where we begin and others end, so that we will recognize when our agency is being abused and vice versa. It's a weird-sounding dynamic but in order to get close to someone, to be vulnerable, maintaining a sense of separation and breaking away from enmeshment can give us the freedom to engage. It can alleviate fears and create stability. I hope that makes sense to you. It's a tough concept to describe. 
