Is swimming very high volume the only way to get better?

Hello, swimmers! I'm barely a real swimmer... I come from a running background (middle distance, mostly, and a bunch of post-college 5ks) and a series of injuries forced me into the pool. I actually couldn't swim at all until 2013, but seemed to improve fairly quickly (at freestyle). So here's my question. Can I keep improving without doing super-long swims? I do not seem to recover or cope well with long swims. I've gone up to 3500 yards in a single workout (took almost an hour), but it basically wipes me out and I don't think I'm good enough at form to keep good form for the whole swim. But when I reduce my workout volume to 1800-2800 yards per workout (but lots of hard sets that a great triathlon coach writes for me) and swim 6 times per week, I do improve! I've brought my 100 yard time from 1:31 down to 1:21 in the past few months and my 500 from 8:00ish, to 7:28. I already have good muscle strength and aerobic fitness from all the lifting and training I did for running so I think mostly it's my form that holds me back in the swim. So should I keep swimming shorter swims and wait for form to smooth out? Or should I push the distance (maybe even just once a week?). Will I EVER feel good going long? One of the issues I have is that my health isn't great . I have an eating disorder history and tend to underfuel and that may be an issue too, though I'm doing much better now and am at a very healthy weight. I feel like my body is kind of exhausted, and while I like swimming, I want to get better at it without overexercising. Also, how do I do a darn flip turn?? I feel like my times would drop if I could do one, but as is, I'm so clumsy at it that they slow me down. Thanks, everyone. I hope to make swimming 'my' sport, even though I only began in my 20s and would love some input.