It is pretty much essential to get your manuscript beta read before finding an editor. I've found that authors who have worked with beta readers before coming to me have a clear idea of what they want their book to become and are excited to get there. It is important, however, to have clear expectations of what they can and can't do for you.
For the most part, beta readers work unpaid. It is common, instead, for another author to read your work in exchange for you reading them. In other cases, beta readers may be avid readers who are keen to be involved in the process of book production.
Beta readers are here to give you macro feedback on your story.
Knowing what to expect can avoid a whole range of stressful or upsetting situations.
Beta readers are readers. They aren't editors.
They shouldn't give you line-by-line feedback on how to 'fix' your book. They should be treated as a focus group. Situations of overly helpful beta readers can be helped by preparing a short form with a selection of questions for readers to provide feedback. This is equally helpful for you when you beta read so as not to take on too much work. Everyone means the best, setting parameters helps you do your best.
When you send your manuscript to be beta read, it is good to have a set of questions that you want them to think about while reading or having read it. These questions could be anything but here are a few to inspire you:
What are your opinions on X character?
How did you find the pacing of the story?
Do you think the plot is believable/plausible?
Did any part of it lose your attention?
What is your favourite part of the story?
Untangle constructive criticism from hurtful comments
Beta readers, like editors and anyone else, shouldn't be mean. Unfortunately, very occasionally people who over beta reading just aren't very kind. Expect constructive criticism, you can't grow without it. But comments that are outright insulting should be disregarded.
If a comment from a beta reader has upset you your immediate response should be to respond thanking them for reading your work and telling them you'll get back to them once you've digested their feedback. Leave it a couple of days and read it again once the initial sting has worn off. It might have been genuinely constructive criticism that you took badly, in which case respond to the beta reader again thanking them for their feedback before thinking about how you can implement the suggestion.
However, it might just be mean. If it is, wash your hands of that reader, pull your socks up, and know that you're better off without them. There are plenty of other beta readers out there who want to celebrate and encourage your work.
Beta readers have lives
If you're going down the route of using writers and avid readers as beta readers then they have no obligation to get your manuscript read by any particular date.
Beta readers are well-meaning and kind people. But they are people. If they disappear there could be a million reasons ranging from personal problems to simply being busy. Like you, they are writers on top of everything else. An editor can respond to your quick question the same working day because it is their job to. Your beta reader has other commitments that may take priority.
If your beta reader does disappear into thin air, a kind message asking if they're okay and letting them know you're in no rush for feedback from them will likely prompt a more desirable response than a worried one asking where they've gone and when they'll finish. If your reader is overwhelmed with life, and then receives a chasing email they might decide the path of least resistance is to 'ghost' you. Which of course wouldn't be professional, but they aren't professionals and this result is likely avoidable with a little understanding.
That being said, some people are just flaky. Good advice when approaching a beta reader is to discuss reasonable expectations of timelines before you both start and to not put all your eggs in one basket. A good number of beta readers to have is 3-5. This ensures balanced responses and also that if one reader flakes out you're not back at square one.
Beta readers are incredibly valuable when used properly. Remember to discuss expectations of both you and your reader before you start. You'll likely develop a great network out of this step in the process that will be supportive and encouraging.