Is the Atheism/Theism belief/disbelief a false dichotomy? are there other options?

[Pyrrho]

I've heard quite commonly, that if you're not a theist, you're an atheist, and vice versa.

That is really a question of definition.  If "atheist" is defined as "not a theist," then it covers absolutely everything that isn't a theist.  If it is defined differently (and there is another common definition), then there may be middle ground.

I've also been told there is no in between, as you either believe something, or you don't, but is that necessarily so?

Yes, you either have a belief or you do not have it.  You might be confused by the fact that humans sometimes have contradictory beliefs, which may make it seem like there is some middle ground when there isn't.

I'm not speaking of agnosticism (defined by one's ability to know whether god exists, which is a different question). 

What if I'm not certain that I believe in one or the other?

One can be uncertain whether one believes something or not.  That does not alter the fact that one either believes it or one does not.

So, one may not know whether one is a theist or an atheist.  But not knowing something does not mean that there is no truth about that something.

I mean, I'm only certain of the experiences that I have. Beyond that, I have no certainty.

Is it because "belief" is an experience, and "lack of belief" another experience, that people say these things?

I would not call belief an experience.  I tend to think of it more as a state of being.

I don't exactly know why there is no middle position, when my experience would accurately be described as "uncertain of belief".
I at times feel as though I am an atheist, and at one point in my life I was a theist, but now, I am completely uncertain of my beliefs.
I don't know if my belief reflects that of theism or atheism, because I don't know if I have enough information to believe in anything, despite this, I manage to maintain some beliefs, such as: 

The belief in an outside world, the belief in other minds, and the belief in a stable, quantifiable universe. 

I'm sorry if this was winding and confusing, but this is the state of affairs in my brain. 
I'm also sorry if this has been asked before, and I've accidentally overlooked it.

What are your thoughts on this?

Okay, so you do not know whether you are an atheist or a theist.  That does not contradict the idea that you must be one or the other.  Right now, I either have some undetected form of cancer, or I do not.  I do not know which applies to me.  But one of them certainly does.