Sometimes I get the feeling like if I don't feel my breathing I'm not doing it right. Simply thinking about your breathing isn't necessarily dangerous. Regardless of whether you think about it or not, you will continue to breathe. It becomes a bit more of an issue if you start breathing too fast for too long because of this. Normal breathing rates for humans is around 12-20 breaths per minute. However most people cant comfortably keep breathing much faster than this for very long and your body will adapt as necessary if your O2 or CO2 levels go out of whack.
I don't know if you have already tried this but when it happens to me I force myself to do something else even for just 1 minute (or 15 or 30 seconds), and then think back, ok so I managed for 1 minute without thinking about it and nothing bad happened. I recognize you said this was a struggle. Even 15 seconds at a time gradually building up may help. Most people can hold their breath for more than 15 seconds so it wont do any damage. I don't know if this is your thing, but things like Sudoku work for me because it engages me visually and gives me something to focus on.
Sometimes this works for me and sometimes it doesn't. Sometimes I just continue on my day and then suddenly notice hey I stopped thinking about it eventually.
I hope things start to improve for you