I've raised quite a number of flags on this site (averaging one about every couple of days) and I can see my "helpful flag" count and see the helpful flags I have raised. However, in order to get better at realising when raising a flag is appropriate, I'd like to see when I have raised unhelpful flags - is there a way to do this?
Yes, you can see them on your your flag history page on the right hand side: it will show counts for disputed or declined flags if you had any. See a snapshot of the flag history page. The decline stats are listed with the other stats on the right sidebar of the page.
Incidentally it's quite normal to have a few declined or disputed flags. If you do get some don't panic! It just means somebody disagreed with your judgment call, which is to be expected now and then. Flags marked disputed are ones were other community members saw had been flagged in a review queue and "counter flagged" (flag to say no action required) and you don't get to know who (except that they have to have > 10k rep to even see that flag queue) or why. In the case of declined flags moderators will usually leave you a specific reason why they decided the flag was not valid. Reviewing these reasons and keeping them in mind for future similar situations is all you need to do.
For reference, even some of our BEST flaggers have ~ 5% decline rates.