Q: As my desire to get closer to God has grown, the intensity of my meditations has grown. The inner space in my head has grown bigger, and there is more of a feeling of "becoming" the mantra. Is this "becoming" real or is it an illusion? 

A: Yes, our desire for spiritual progress (bhakti) has a direct effect not only on our regularity of doing sitting practices, but also on the results. If our desire is very intense, we may, in fact, do well to "self-pace" our bhakti just as we would any other practice to avoid excesses of energy flow in the nervous system, which can be uncomfortable and counter-productive to our progress. If our path is unstable, we might not be so eager to stay on it long term. What we'd like is good progress at a smooth, safe speed. That is a path we can stay on for a long time, which greatly enhances our ability to achieve maximum progress in this lifetime.  

On "becoming the mantra" in deep meditation and whether this is an illusion or not, some might argue that there is no "becoming," and that we are there already. However, while we'd all like to "Be here now," the becoming-ness of it is impossible to avoid. It is a matter of changing point of view as our nervous system progresses in its process of purification as the vehicle of our inner awareness. If I am out here in ego and go in on the mantra, I may appear to become the mantra as it and my egoic sense dissolve into stillness, which is my self. If I am in there already as resident inner silence, the witness, looking from the inside out, then there is no becoming as the mantra makes the same journey. The process is the same. Only the point of view is different. In the case of your experience, you have seen your ego self dissolve to become the mantra refining into expanding stillness, your unbounded blissful self. There's nothing wrong with that experience, as long as it is only an observation of scenery and not an imposition on the easy procedure of deep meditation itself.  

Of course, we cannot intellectually locate ourselves in one point of view or the other -- not organically anyway. Our point of view from ego or stillness is where it is depending on the condition of our neurobiology, and we will describe the experience from that angle accordingly. With practices, over time, the point of view gradually shifts as our sense of self changes from external to internal. Then we can honestly say, "Hey everyone, I'm really here now!"  

In truth, we probably won't say much, and just go do the dishes and take out the trash. Nothing changes, even as everything does. We are becoming "That" even though we already are "That." I guess that is why they call it "realization" ... realizing something that is already. It is a becoming ...  a journey from here to here...

The guru is in you.