You aren't a Boltzmann brain.
Sean Carroll wrote a blog post about this that you should read. It's funny, and gets its point across well.
The key point is that it's a premise of the Boltzmann brain argument that we aren't Boltzmann brains, so it makes no sense to conclude from the argument that we are.
How do we know that we aren't? I think the argument for that goes something like this.
Complex systems can arise by random chance, but the probability is low. The more complex the system, the lower the probability.
The probability of a brain randomly appearing in some location is not zero, but it's ludicrously smaller than the probability of that location containing, say, hydrogen gas.
The probability of a brain and a surrounding body and ecosystem and fake fossils suggesting that the brain actually evolved through natural selection appearing randomly is ludicrously smaller than the probability of just a brain appearing randomly, never mind the hydrogen gas.
Nonetheless, you can construct cosmological models so ludicrously large that the chance of the brain + fake evidence appearing somewhere is pretty high.
The thing is, though, that if the model is large enough for even one random brain + fake evidence to exist, it should also contain a huge number, an absolutely ludicrously huge number, of random brains without the evidence, because that's so much more likely. So, if you're a random brain, then with ludicrously high probability you should not see evidence of evolution. But you do. This is a good reason to be ludicrously confident that you aren't a random brain.