Whenever I hear "harsh", I equate it to alcohol (when I'm reading DIY stuff). Though not always the case (as sometimes a poor vg or pg or even nic can be the problem) most often IMO it's the fact that either the new person is unaware of "the alcohol factor, or forgot to consider it.
I would be willing to bet that there's nothing wrong with anything, and that it's just part of the learning curve that comes with learning to DIY mix.
Some flavors have more alcohol than others, and some have no alcohol at all. Given the variances, you'll have to learn (and make note of) which flavors you need to breathe, and for how long.
After you've mixed a batch (especially if you use heat, you need to let it cool to room temperature) you'll need to remove the cap and let it breathe for anywhere from 1 to 24 hours (varies depending on manufacturer, and particular flavor -even when the same company is used).
Good luck, and welcome to ELR!