-1. BIG ISSUES
These are the gaping plot issues. It may be as simple as when you are writing the first draft you got to a fight scene and wrote in big print "INSERT FIGHT SCENE HERE. MC WINS." Well, guess what? Now you have to write the scene. It may be you change your magic system, need your boyfriend to have a different job than he had the first five chapters, or perhaps you discover you need to tell the story from 3rd person point of view instead of in your character's head. These issues involve major rewrites
When I wrote my first book, Jam Sessions, I got to the end and was overjoyed. I had written a book! Well, I had written a draft! I was ready to go to press, right? No. My main character moved schools in the middle of the year. On his first day he is taken to class by the school bully who subsequently becomes his friend. That is until, the MC is bullied and teased. My MC must find a new friend group and a way to work through his panic attacks. That was my book. I had a whole backstory about why he was in a new school. It was interwoven into the book with a lot of "not in school" scenes. It made the book darker (an abusive dad) and lost focus on my MC finding his way amongst his peers. Beyond that, it was a conflict already resolved (maybe not fixed, but resolved) when the opening chapter arrives. Simon (my MC) and his mom had moved states and were living near Simon's grandparents. I went through teh book and cut all my beloved passages. I rewrote the start of the book. It showed a sleepy kid being woken at 2:00 am as his mom drove away from their house. It sets the stage without being a book about trauma in the home. That's a good book, but not the one I was trying to write. My intro isn't on par with the best openers on the market today, but it is a solid freshman effort because I was willing to radically rewrite sections of the book.
However you decide to do it... DO NOT... I repeat DO NOT start sending your first draft out to agents yet!!! Take some time to correct and revise. If an agent is something you want, it should be pursued with the cleanest manuscript you can offer. So, what are your options to fix the major issues?
Do it your self - If you choose to do this yourself you MUST walk away from the book for a while. Get some distance. Start a new project, read, go reconnect with friends and family you neglected as you did the final writing sprint to finish. Six weeks to six months. I would not suggest less or more. Then come back and read the book. Do not edit. Read it through and annotate sections that need work. Again, don't fix them. Just identify them. That way you are reading the book all the way through in one pass. It is the best way to catch the big issues.
Pay someone to do it. - Paying someone to beta read is fairly expensive on sites like Upwork and Redsy. However, there are folk out there you can subcontract out for the work. I think better are groups who love ARC's (Advance Reader Copies) and book bloggers. You can find ARC readers on sites such as Netgalley. They will read and offer feedback. The good part about such sites is you can join with a group of people (network!!) and it has great promotion capability also.
A word on paying someone. Don't be prideful about this. In fact, embrace it. For me, writing is a hobby. I spend much less on it than my brother does on hunting and flyfishing. It costs less than my neighbor's obsession with buying souped up trucks and atv's. Today, my hobby even pays me a little bit back. Whether you are doing it as a future income stream ("gotta spend money to make money") or as a personal journey, it is okay to spend. Just spend in the areas you need it the most. Set a budget (more on this later) and follow it, just like you would if you got heavily involved in golfing, scrapbooking, backpacking, or aquatic crocheting.
Coerce, Bribe, or Beg someone to help.
Ahhh the Beta reader. This is not your mom telling you how special you are and posting your book cover on the fridge. Don't get me wrong, that is important. But here we are talking about that unique person who loves (1) to read (and has read a lot, especially in your genre), (2) likes you and is willing to read an unpolished copy, and (3) is willing to take the time and energy and risk to offer correction.
My first set of Beta readers I asked ten or so people. six said yes and never read or got back with me other than the occassional, "I haven't forgotten!" This is true. They hadn't forgotten. They just weren't all three of the things above. Of the other four, two sent my manuscript back telling me how great I was. Kind, but unhelpful. One spent lots of time correcting small grammatical issues. Again, it was kind, but by the time I rewrote the draft these corrections were often mute.
One sent my manuscript annotated as another author asked her to do. She was awesome. Here is her list:
"ZZZ" meant the story did not hold my interest.
"???" means I am disoriented, confused, or see something that doesn't match. These often had comments attached.
"!!!" meant I loved this section.
A solid line "--------" meant she stopped reading here.
In the end she spoke to these areas:
-1. Characters - Were they believable? Could I identify with them, especially the MC?
-2. World - Was the world / setting understandable? Interesting? Coherent?
-3. Plot - Were there plot points that were unbelievable or you had trouble with?
-4. Dialogue - Did each character have his/her own "voice?" Was dialogue used well? Sound artificial?
-5. Show, Don't Tell - Were there areas you felt like I was teaching, lecturing, preaching rather than immersing you in the story?
You can google "Questions for Beta Readers" and come up with hundreds more. My suggestion, keep it simple so the reader can annotate as they read with a very brief overall assessment at the end. i've seen some Beta reader questions that emphasize the first chapter too. Since this often gets sent to agents it makes sense.
I should mention I discuss the Beta Reader as a free option. You can pay Beta Readers as well, though if I was going to spend that money I would get someone more professional on the content editor status. Also, in the realm of advice, if you "pay your dues" in a writer's group by being active, helping others, and generally not being an ass you may find some folks in that circle who will swap ARC (advance reader copies) with you.