Breathing a mix should only be done in short bursts, not days.
It's not unusual to "taste" a flavor initially, and then have it change into something unvapable....Then something really "great". That initial "taste" well, if it doesn't linger you need to just steep. At least a week, shaking maybe once a day.
I don't even taste my initial testers at the beginning. I've been doing DIY long enough to know that there is little point, unless it is specifically a "shake and
vape." I enjoy most of my mixes at the age of 3 weeks or older. It can be tough starting out to make sure that liquids get "aged" enough. I'd suggest sticking to storebought for a while, and not tasting any new flavors for at least a week, and I would not be surprised if they continued to improve after that time.
In my opinion (everyone is different) there is no substitute for time. I do not heat steep, I don't agitate my mixes much other than shaking them vigorously from time to time. You just can't really "speed" the process much, has been my experience. Occasionally, I will do a "quick and dirty" heat steep for someone I am mixing for, but I still caution them to wait at least a week, if they can. Or I use really bold out of the gate flavors as primary, I'm talking stuff like blue raspberry or forest fruit.
Best of luck, patience will be your best friend here.
Anna