"It's come to my attention that, maybe, my identity is nothing more then what I've built piece by piece with various beliefs;"
Bingo. All it's missing is the template nature you were born with that served to alter the odds of what beliefs you follow, how you follow them, and why.
"I've cobbled it together from views of spirituality, sex, gender, morals (or lack of), and motivations."
You make it sound like you had a choice in the matter with the phrase "cobbled". Most things about you are thrown at you, not truly chosen. It's all about how it's presentation spoke to you and when it did.
"Even what's there though, right down to the foundation, is loosely stacked together like a game of jenga block, because I remove and place blocks of my identity to suit my needs, in whatever way is convenient as the game progresses."
So do most people. The only problem with this sort of realization is a potential lack of confidence behind yourself, your sense of identity, which for many can be crippled by the awareness of this sort of thing. Denial or naivety tend to reflect more zeal.
"So what then do I do when that is challenged, when someone else walks into my life and topples my tower?"
Use better building materials. Just because it's a tower doesn't mean that it has to be so fragile.
"I realize my immediate response, just like anyone else, would be to prevent this from happening by aggressively defending it."
Why is that, in your case specifically?
"I've come to realize though, that it is a battle for defending my ego, and that ego should not take precedence over accepting the raw reality that the ego matters not"
Nothing matters, so your ego is on an even playing field. Why not have it take precedence over anything else?
"regardless of whatever metaphor of that toppling force is, the blocks are always there to build with."
Seriously, you ought to consider some less fragile building materials if you're constantly worried about toppling. I see mine more like a series of wires that all have the potential to connect to one another from being made from similar enough materials. When most things are dissected far enough, they tend to be made of smaller pieces that intersect into many more areas than they do if left as they were. Sure something might go wrong and I'll find myself short circuiting, but when I find the culprit I have that much more room to reinforce the design.
"as early as I can remember I've struggled with a sense of self, an identity."
Why struggle? Identity forms itself.
"I couldn't tell who I was, other people were thrown off because they couldn't tell either."
Good, keep them guessing. Predictable people are less interesting.
"At the end of the day though, I still am that identity shapeshifter that only changes when no one is looking."
What do you do when people from different settings meet up with you at the same time then?
the majority does.
it doesn't make sense, because there are things that can't be proven.
but it makes sense that the majority knows better in terms of the basics: what is reality.
you can't claim that we are in the matrix, unless you have evidence, because this is part of the basics and we get our information on the basics from the most reliable source(not a 100% reliability, but it is the most reliable one): the majority of humans and their opinions and how they perceive things.
And from there on, we can make facts, based on the basics of what reality is, that are more reliable, then the opinion of a majority, and do not need to be accepted by the majority to be true. Because the majority can see the basics well enough, but not everyone can see beyond them so well, so you can't do it the same way there(aka even if everyone say there is a God, that doesn't mean its true just because everyone say it).