Hi Sorehead, I too learned on cruiser style bikes many years ago and then "graduated" to sportbikes about three years back.
I think that I also understand your problem. Very tight cornrs were my problem too, still are I suppose. I was quite comfy in a higher speed corner, the bike feels stable and you just have to hang on and go with it.
On low speed corners I felt wobbly, and when I recently did an adavnced course I realised that I had been riding in 2 modes; fast proper riding, and a stupid "wobble around the tight corner like it is a driveway" mode.
In the advanced couse they taught us to lean off the bike at high speeds (we all know that), BUT also at low speeds (under 15mph - most street corners and intersections) you lean the other way. I mean lean the bike over, but YOU make an effort to stay up on top of it.
The instructor did an impressive demo on a big 1100cc BMW, he got on, laid it over until the peg was almost touching (with him stood up on top of it), gunned the throttle and spun around in tight circles doing about 20mph!! I couldn't believe it! Looked like a circus act.
Then they set up a very tight course with little witches hats, and we did laps around it and it was great fun! First time I had ever enjoyed leaning my bike right over at "parking lot" speeds.
Since then I made a conscious effort to fix this weak area in my riding, and it is amazing how much I have improved (or how BAD I must have been!)
Just try (only in slow corners, u-turns, intersections etc) leaning the bike under you but staying up on top of it. Almost instantly it feels "right" and you have more control and confidence. Hope this helps.