When sub-ohming (or large direct lung hitting) there's a much higher saturation level of vaper to the lungs, this is why we need to drop the nicotine levels. If 6mg is too strong drop down to 3mg, you'll probably be more comfortable. For now, with the 6mg juice lower your wattage a little and take smaller hits (don't go beast mode), also space the time between hits and you should be ok.
For example, atm I'm waiting on supplies, I'm left with some 12mg juice here (usually vape 8mg). Long as I keep the wattage lower than normal, take little smoke size puffs and space them out I can remain quite comfortable. Ultimately, you want to get your nic dialed in to a point you're enjoying vaping, getting your nic hit and using juice efficiently.
Another example would be one person I saw vaping 120mls juice per day. While watching his video I noticed he was chain vaping the entire time, huge big direct lung hits. I later asked him what level nic he was using, he said "1mg". In my opinion this is silly because he's spending the entire day chasing his nic hit while chewing through ridiculous amounts of juice. The person would be much better off on 3mg and consuming 70% less juice per day.