very disappointed about my progress

I took my 1500 m time in a 50 m pool 12 weeks before, and it was 31'42". Afterwards the 50 m pool I use closed for annual maintenance. In these 12 weeks, I joined a squad and did 2 sessions per week, swam open water with a group every weekend, and also swam intervals on my own in a 25 m pool, with about 12 - 15 km total per week. The 50 m pool I use reopened this week, so the first thing I did while returning was to take my time again, but the result was 31'13" - only 29 seconds improvement over 12 weeks! In contrast, I improved for 45 seconds in the preceding 6 weeks, from 32'27" to 31'42", and those 6 weeks were the weeks I newly joined the squad and I normally did only 1 session per week, and no intervals on my own. I was targeting 30' and attempted to swim at an aerobic intensity which I thought I could sustain for 30 lengths, but I could only sustain my form for around 400 m. Afterwards, I felt my teres major were so tight that I couldn't perform my stroke well, my arms simply failed to execute my desired stroke even I reduced my stroke rate a bit, and the stiffness even persisted in the cool down after completing the set, and for another few hours. According to my wearable, I was taking 31 - 32 cycles at the beginning, and 34 - 35 cycles at the end per 50 m length, while my stroke rate remained mostly the same. I was swimming at around 59" / 50 m in the 2nd to 4th lengths, then deteriorated to around 62" / 50 m in the middle, and about 65" / 50 m by the end. The cumulative time as recorded by my wearable was follows: 250 m: 4'52" 500 m: 9'59" 750 m: 15'19" 1000 m: 20'31" 1250 m: 25'56" 1500 m: 31'14" I'm very disappointed that, even after a few months of intense training, I still haven't reached 30' / 1500 m in long course yet. I'm taking a very expensive 1-1 professional analysis tomorrow, and hope that afterwards I can know all my technique problems and target them in the coming month in my own training, and get a jump in my speed.