Dave, there are students in our dojo who still have problems with the correct tsuka grip after six years of training (I've been assisting Sensei at least that long with both of them), so don't despair if you don't have it quite right after two whole months.
The major trick I've found is to relax your grip a whole lot more than you would expect. Those articles that Kim wrote explain that, but it's not at all obvious at first reading - or practicing. Of course you don't want to drop your iaito (or, worse, fling it across the dojo at sensei!), but if you try to see just how "lightly" you can make your fingers hold onto the tsuka, you'll be on the right path.
Charles is right that tenouchi is the key to control, and different iaido ryu practice it differently. By the way, you didn't mention which ryu your sensei teaches - that may give some of us a better idea of what to suggest. I wrote an article on hand control for the dojo some years back, Dave, and I'll extract a bit for you here (we practice Muso Jikiden Eishin-Ryu):
Generally, the grip should be strong in the bottom two left fingers, although I've heard a theory that the middle finger can also be held firmly. In terms of kiritsuke, it's essential to maintain a firm grip with the little finger of the left hand at the beginning of the cut. This ensures that the kissaki moves at the
beginning of the cutting action, and doesn't delay movement for even a moment. All too often, the hands or wrists are moved first, leaving the kissaki behind, but this is incorrect. It's important to have a strong grip during nukitsuke, provided the grip is correct. The tsuka should be well-seated in the base of the right hand and, generally, the line along the top of the hand to the forearm should be straight; i.e., no bending the wrist forward. This maintains the correct relationship between the tsuka and the bottom of the arm. This relationship/position should stay the same no matter in which direction a cut is made. It is, by the way, the same position as when completing chiburui.
The only place for tension or a strong grip during kiritsuke is really at the point of applying tenouchi. Before and after this point, the grip should be relaxed. In so doing, the sword will swing naturally with the force provided through the body (and gravity). Any tension applied to the blade through the hands/grip will cause it to deviate from its intended path. That's the main way single- and double-handed cuts differ. In nukitsuke, being a single-handed cut, the tsuka by definition must be held more firmly to be effective.
Have I confused you, Dave?
I have full video equipment & editing capability if we can come up with a plan.
years of practice! Seriously, how you handle your iaito is an evolving process. I had one hachidan sensei tell me that if I ever did any waza exactly the same way twice, that I was stagnating & beginning to fall behind. It took me a few years to figure out just how right he was!