I first got into a pool 11 months ago and like you I could only manage a few lengths without being totally exhausted despite been pretty fit.
I got a couple of half hour lessons focusing in on slowing down my kick, constanly breathing outwater and little bit on hand placement.
I was good enough to get me through a sprint tri and then progress to a 1.9Km in a half iron man.
But I wasn’t really progressing so I went on a TI day workshop a few weeks ago. I tried to put everything in place at once and I sank.
This week I focussed on head position and saw some gains for the first time in months. I’m not sure the workshop was worth it but I’d definitely recommend getting some coaching or going to local masters class. In my area they cost the same as normal swimming session but you get some coaching thrown in