Hi Steven, since I came together with the woman who is now my wife, I did everything in open discussion with her; from discussing their role in our relationship to choosing compression shirts to everything regarding my recent surgery.
I think what is important is that they are an (integral!) part of you. It is something you cannot get around, along with their downsides (e.g., the sores). You write that she is very understandable, so I am sure that she will be open to reason. So I would recommend you to sit down with her, and have a talk in which you:
- Acknowledge that she has her problems with you wearing bras.
- Explain that you want to make your point clear.
- Describe your discomfort and your fear of not doing anything about it.
- Provide supporting arguments (sagging breasts is definitely one of them).
- Try to understand what feelings still cause her to not be okay with it.
- Find a solution that respects her boundaries but meets your needs.
For me, a core aspect would be to understand why she has issues with it, and why she tells you: "just let me know ahead of time so I am prepared for it". I cannot quite understand where that comes from, but I am convinced she has her reasons. Maybe it is a form of implicit disgust, where she feels very confronted with your breasts when they are so clearly "marked" through a bra. I know you have very good reasons, but I'm sure she has very good reasons as well.
If you can find that out, maybe the solution could even be that she just has to get to terms with you having to wear a bra, and maybe the both of you just need a transition period in which you slowly increase the frequency and duration of wearing a bra. You can work on it together and keep it a point of discussion.
If she has a problem with bras but would not be as put off by a compression shirt or other garment that relieves your discomfort, that might be a good compromise during the times that you are with her.
If there is no possibility for a solution, an absolute
last resort would be to use a "veto". You are entitled to it, just as you are entitled to a band-aid to put on a wound or a concealment if you have something embarrassing (e.g., a pimple or ugly scar). You have sores, which is your body telling you that
have to do something about it (and things could get even worse). I would still recommend taking the "discussion" road, but if this is a point where she is not as understandable as usual and stuck in a way that you cannot get her out of, you'll have to persist. But perhaps I would even prefer a joint visit to your GP or to a mediator first, as this is a big thing.