I'll start with an aside, that I'll come back to at the end:
I am intimidated by the complexity required of quality fictional narritives [sic]. One of the elements of fiction, the three-act structure, seems like it takes some time to memorize.
While I personally don't see how the three-act structure takes any time to memorise at all (it is, as the name implies, only three parts: Setup, Confrontation, and Resolution), the fact is that you don't need to memorise it. Just write it down somewhere, or bookmark the Wikipedia page or something, and refer back to it anytime you need to.
Have any of you felt intimidated by the complexity of fiction when you first started writing it?
Not so much the complexity, more the size. It's easy (or at least, I personally find it easy) to come up with a compelling story idea and the basic plot beats: A happens, then B, then C. Coming up with the material needed to link A, B, and C together, and ensuring that it's all meaningful or enjoyable in some way? That's considerably harder. Sometimes I can't think of a way to get from A to B; other times I think of a hundred different ways and get choice paralysis.
The way to combat that, of course, is to just write shorter stories. If writing a novel seems too complex, try writing a novella or a novelette instead. The shorter length gives you less room for the kinds of subplots and large casts that more complex narratives have, and keeps your narrative simple and taut.
Personally, I find a lot of the complexity in my writing arises organically during the writing process, as I find ways to weave in subplots that I wasn't sure how to integrate, or spin off new subplots I hadn't thought of before, or invent new characters (or flesh out existing ones) to fill roles that I hadn't foreseen.
And can one still be a writer if they feel this way?
Of course! You yourself have stated that you've written "simple stories" in the past despite your own struggles, which means you're already a writer. There are plenty of tools and methods out there for helping writers keep track of their narrative and manage its complexity.
If someone feels this way, does it mean that they lack the necessary intelligence to write fiction?
No. Managing complexity isn't a matter of intelligence but skill. Skills can be practiced, and as stated above, there are tools and methods available to help you.
I feel like you're trying to memorise your story's entire narrative structure, and that's why you're struggling. You don't need to do that, just as you don't need to memorise how the three-act structure works. Write it down. Even just scribbling some notes can help you immensely.