Oh, bloody HVs (sorry to the good ones - I know you're out there)! Hasn't she come across the WHO guidelines yet?
I don't think she could show you any evidence that it's a 'normal thing that comes with time'? Your body does change over your breastfeeding life, however. Have you been well recently? Sometimes colds, viruses, other life changes can affect your body and as breasts are attached it could affect them too!
You're right, relaxing can help but it's very hard to relax on demand, and knowing you have to relax can make you anxious, iyswim! You could also try:
Gentle massage before feeding - if you go to kellymom.com there are good instructions for manual expression, and you could use the first few stages of that to see if it helps.
Warmth - tried feeding in the bath recently? Putting a warm flannel or a hot water bottle on your breast before feeding?
Visualisation - close your eyes for a couple of minutes before feeding, and imagine yourself looking down at your babe feeding
Leaning forward a bit to help with flow (lots of cushions behind the back)
Changing position - perhaps as your DD has grown her latch has changed and your currently feeding position isn't encouraging as good a latch as it used to?
These are all good for daytime but not as much help at night, of course.
My DD fed to sleep until she was about 20 months, then stopped spontaneously. If it works for you, ignore the HVs dire warnings (which aren't based on anything other than the conventions of the last 100 years or so). I don't follow AP religiously, but have an extremely independent, confident, sociable nearly 3yr old who co-sleeps, breastfeeds and was carried loads. So yarboo sucks to all those who said I was creating a clingy, fearful child. It seems in my highly biased opinion that if you let a child fulfill their need for contact, cuddles, babyhood in general, they can move on at their own pace (assuming you're able to do that, and I do understand it's not always possible) and it seems to be very good for them emotionally. Your daughter is 15 months old. She isn't independent and isn't supposed to be. She's still a baby! The feeding to sleep thing is only a problem if it's a problem for one of you.
Sorry, that was a bit of a rant .
As for work - only you know if it's stressing you out. It would be very hard to prove cause and effect. Good luck in trying out a few things and perhaps it's worth talking to a BFC?