Welcome here, and thank you for giving such comprehensive advice to AshIII a while ago.
Having read your post I get them impression you have two problems. The condition of your partner -and also your own.
You may think you have a great many reserves of strength and can bear part of your partner's load when his symptoms are more pronounced, however it is a drain on you - often hidden but there nevertheless.
I have, among other matters, an anxiety condition and when it was at its worst my wife had great difficulties in knowing waht to do, the uncertainties and worry over the relationship and if she was doing right or at fault. On top of htat my moods made her feel worse.
When here mum came over thngs changed, my wife's stress levels went down, further helped by explanations from my doctor as to what were the expected symptoms.
It is easy to drain yourself for someone you love. May I ask if you think your current medical support is doing the job? Franky I think your phrase:
"it's hard to know for myself where the line is between his feelings and mine." makes me worry for you.
I'd also like to ask if you have the same sort of personal support my wife had? A family member or freind perhaps who cares and will listen and offer perspective and maybe even practical help. This can make a huge difference.
With you partner, as in my case, I would think the single best thing you can do is to persuade him to seek full medical treatment, which may be both medications and therapy, perhaps wiht a specialist psychologist or psychiatrist. Health anxiety can be hard to reduce, each minor bodily change bringing up fresh fears.
How you do that I'm not sure, perhaps you have some suggestions as the person in the middle?
To allow your partner not to feel excessive worry or panic may take time, but like most things the outlook is good with the correct treatment.
I look forward to hearing from you again