I have suffered with the same so I know what you’re going through and agree, fresh air always helps.
First, do you have other coping techniques or things that help when it happens too? Breathing exercises, cold water, meditation or CBT, meds, sensory simulators such as oils, strong mints etc? Having a go to response is great for reassurance, but you want to make sure that you have an alternative you can practise in case eg you can’t get fresh air.
Basically what I’m saying is don’t let the thought that you can’t get fresh air make you panic more (or bring it on in the first place).
Second, if you’ve noticed a sudden change then it’s worthwhile rerunning tests at doctors so that when it happens, you can tell yourself it’s “just” anxiety and that actually everything is physically ok.
Thirdly, as for your DP, HWVVU to say that. However, if I may also say from experience, it will be affecting him. He will be constantly aware of your worry even if he doesn’t know when it happens. Maybe it’s worth sitting down and asking whether he does feel it hasaffected anything, at the same time as explaining how changes are affecting you. Maybe not bottling it up will help.
Good luck I might go and sit in garden now