I hate hate hate having underinflated tires. They suck, quite literally. It feels like trying to pedal a boat when cornering, all soft and wobbly. So my bet would be for medium inflation (funny phrase ) and handling practice first on a flat, firm surface, to feel what it's "supposed" to be like, and then on a looser surface, to get used to what it can be like. You'll probably feel a difference according to your speed as well, more stable the faster you go.

And thenyou might have a stem (is that the right word? the thingy that your handlebars go through) the wrong length. I've tried different stems, and some of them made me feel impossibly wobbly.