Here is a smart wearable device that monitors your breathing and gives you feedback. The Oxa Bra/Shirt comes with a compact sensor that connects to your phone to monitor your breathing and give you real-time data and feedback. Oxa measures breathing rate and volume by a process called respiratory inductance plethysmography. It can also read your heart rate and HRV.
The Oxa lasts up to 24 hours on battery. Its app is available for iOS and Android devices. You can use this breath training system with Strava, Google Fit, and Apple Health. The bra / shirt are available in S to XL sizes.