Doubts are normal. Forcing yourself to not doubt would make you go against something that's normal and probably make yourself doubt even more.
But until you are certain, there is no reason to make a decision. Perhaps you are depressed and that causes the feeling, instead of it being the other way around. That was everything could seem off, so even trying to make decisions like was it all a lie just sounds something that not's very well thought out. Suicidal people rarely think their suicide through, either. And then after their try, many do want to live after all.
I guess the question is, what did you believe in her now that you are doubting? Kinda implies there had to be something you believed in for you to doubt now. Her speech? Perhaps something else? I would suggest you try telling her to do things you can do and then you trying to copy them. Look very carefully at everything, feel every little difference. Even if something feels like it's 99% same, that 1% came from somewhere. If it happens all the time and you have to like say, use more conscious thoughts to parrot things, then maybe there's something very special in that 1%. Something would be different, the question is why. Maybe a tulpa? But if it's same, it's hard to say if it is the same or if you just haven't seen the difference. Not a very definite way to do anything like nothing about tuppers is, but perhaps you will learn something.
Young tuppers tend to be very connected to their hosts in like, so many ways. You could easily guess her answers because you just know her that well. You could also get her answer sorta bleed over to you before she actually says it, that does seem to happen. And man, I remember like starting to ask a question and then getting an answer back before I had even finished. It takes less time to think up questions and answers, so once those bleed over... They wouldn't even have to answer with words. Often both of you do, but I had to actually inform the tupper to not talk over me and let me actually finish the question because that was nicer. Weird little things.
The tupper journey is so personal. You two might still be way too early in the process to be hoping for things like speech out of absolutely nowhere which probably would make you feel better, but some people could get that very fast.
Oh and I forgot, when it comes to visualizing, sight was your worst sense? What about other imaginary senses, I think you were probably one of those who had good audio but there's still things like touch and smell. See if you can use them to bring out the visual part of imagination.
