Awesomeness. How you explain machine learning principles/NEAT really shows a firm grasp. The fact it's done in lua...makes it even more eccentrically cool.
You go to "war" with the army you have not the army you want. if you want to get a new founder on board then start looking but I wouldnt delay today because of it.
If you are having second doubts. Then reevaluate. Ask why she left and get More feedback from others.
People will start joining when you least need them ( when you start becoming successful ). So I'd advance only if this is your vision for your contribution to this world.
If I were you. I'd go and apply and start searching immediately for another team mate.
Ask yourself. What's the worse that can happen?
I have no xp with y combinator or all of its criteria but a lot of people view one founder as a weakness (this can also bleed over with angel investors too from what I hear). Fortunately you have the tech side. If you want another founder then start searching. If you want a business side person it shouldn't be too hard.
I personally have started as a single cofounder and raised money by the users. Its not nearly as fun but it's doable. If you believe in it i wouldn't let anyone stop me.