I am sorry but I do think you missed the time to trim.
Trimming must be done RIGHT after they bloom.
1/3 down and 1/3 out is the general rule. If the bush is 6' tall then remove 2/+ feet from the height. If you have 6 Large stems take out 2 of them along with shaping
If you trim now after it has started its new growth, you will trim off the buds for next year and will not get any blooms next spring. I trimmed mine back in May and now have about 2/3' of new growth and will not touch it until next year after bloom. Sorry

Not sure what you're sorry about. I had no intention of trimming in the dead of summer, and an a bit miffed I need to do it at all. My other Lavender bushes are thick and round (example shown from a few weeks back), and I won't ever touch them as long as that holds true. Dunno why this one got so ugly. In any case, thanks for the info, I shall trim in spring rather than autumn.