I just wanted to pop in and give you some encouragement. Yes, as you know stress, anxiety and adrenaline can interfere with all the love hormones of labour. BUT I promise you will not be pregnant forever! You will go into labour. If you find a way to limit some of your anxiety it may happen a little bit quicker but one way or another it will happen. It actually already IS happening! Everyone's labour is different (and it almost never happens like in the movies). I get a very strong feeling that your baby and your body are just giving you a long slow prelabour to give you a chance to get ready. You are being given the chance to prep yourself just a little bit more. Maybe you need a day or two more to prep for labour or maybe your baby is just giving you time to paint your toenails and fit the car seat!!!
I know you say you struggle with your breathing. It must be frustrating for everyone to keep saying you can do it when you feel like you can't but it's just because we all have faith in you. Can you practice relaxed breathing when you are not anxious? I used to practice first thing in the morning. Maybe try that. Or just forget the breathing and forcing yourself to relax. Just do relaxing things! Swimming, yoga, massage, foot rub from your partner. My labour started after a nice day spent just hanging out with my DH. He cooked me dinner and made me feel very cared for.... Then bang! Labour! But I will bet $10 it won't happen on your due date.... I'll cross my fingers for you it will be the day after.