Beware of reading too much into rejections, even personalized ones. This is a very nice rejection, and you should give it the time they gave to you in writing it, but not more. There's never any point in worrying about whether an editor has actually read your MS, for instance. It honestly does not matter.
That said--and I'm about to ignore my own advice and read into it--I'd guess the subtext here is "you're not ready" which indeed does happen to everyone, but it's likely a larger publisher or agent would form reject you; some newer publishers can afford to be nicer because they may have a bit more leeway with their time, at least at first. Or you just got them on a nice, sunny afternoon with a gentle breeze.
Developmental editing vs. beta-reading vs. classes and workshops vs. reading critically vs. just getting better over time is a slightly different debate, and, IMO, no publisher can or should really recommend one over the other. The takeaway is to realize that you need more time and find whichever of the above (and more) works for you and your book. (E.g., lit-fic tends to benefit more from workshops; it's easier to find betas for SF and commercial, etc. But even these are generalizations.)
I think your initial instinct, that you subbed too soon, is the one you should listen to and learn from. For starters, you submitted to publishers, and while there's no particular
problem with this, if you're aiming for trade publication it might be better to at least consider an agent, and the good news is you haven't burned any agents yet, or any imprints that only take agented subs, which is most of them. The submission process, too, takes slowing down and
learning.
It's likely that you're making, if not
basic mistakes of craft and mechanics than the slightly-more-advanced mistakes that mark you as a promising writer, but one who's very, very green. If you're not familiar with it already,
Slushkiller offers a good spectrum. It's relatively easy to get into the upper half of it--anyone who has finished a book has at least a decent chance of having written functional paragraphs. The later stages, though--that's what separates "publishable" from not. Getting to that level is a lot harder than just writing a book, so as others have said, it's likely this is where your work begins.
A good start would be to get your 50 posts and post an excerpt in SYW, as people here can and will catch the intermediate/advanced mechanical wobbles. The best way to get to 50 posts--and by far more helpful for learning craft than actually posting your own material--is to critique other excerpts posted there, training your eye to catch mistakes that jar your reading eye out of the story. That way you start training your eye to catch (some) of these in your own work. It takes time, but you have to train yourself to get distance.
I've always suspected that an awful lot of people seek out editing way, way too early. For starters, there's no replacement for learning the above. More, a good editor (and there are absolutely
bad editors out there, too, especially among those who will take an unpublished debut MS) will tell you what's wrong, but not how to fix it effectively; a good edit can be and almost always is overwhelming, and if the book really is not quite there, it's just as often the case that the edit can only reveal that a book needs a complete rethink, well beyond what even a developmental editor can realistically provide. It may be the author needs to reconsider why they wrote the book, what the book really is, etc., and no editor can tell you that. You have to learn how to ask those questions of yourself, before subbing, because even once you get the holy grail of a publishing contract, you still have to learn to do this work, except by that point you're under much more pressure and a much tighter deadline.
The easiest mistake to make is to rush. Nothing in publishing moves quickly, least of all authors. Learn to accept that, take your time, and make sure you only ever publish your best work.