Your brain is a terrible proofreader for your own prose, because it silently repairs every typo as it reads. You know what the paragraph was supposed to say, so that is what you see. An editor does not have that curse. They read what is actually on the page, and they tell you where it stalls, wanders or lies.
There are three different jobs here, and conflating them is how writers waste months. A developmental editor interrogates structure — does the story work, does the argument hold, does the middle sag. A line editor works sentence by sentence on rhythm, clarity and voice. A proofreader catches the comma. Hiring a proofreader to fix a structural problem is like painting over damp.
If your budget stretches to only one pass, buy development. A beautiful sentence in a book that does not work is a wasted sentence. And when the edits come back, sit with them for a day before replying — the defensiveness fades, and the useful parts remain.