these statements from your post stand out to me and suggest that you need to become more relaxed and confident in the water >>>>>
but never in water i couldnt stand up in.
then lost the confidence to even do the basic breastroke.
more confident in the water again in shallwo water,
i suppose i dont feel too relaxed either, quite tense
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I agree with all of the suggestions above but would like to add that you need to become more relaxed and confident that you WILL float in any depth of water....your body will be mostly submerged, but you WILL FLOAT... look at the floats in this video https://www.youtube.com/watch?v=-jMn01H6cAA
to breathe effortessly you need to become relaxed but you can't be relaxed if your are at all tense
to run you must first walk, before that you must crawl, but before that you have to be able to roll from your back to your belly....swimming is similar but more difficult for humans...
parse the problem and work on the pieces individually....float on your back to get the breathing issue out of the picture
finally you might try another instructor to see if you can get a better connection to learning the skills
I encourage you not to quit