I'm pretty sure I've said this before, but editing really is your friend.
I would never have believed it a year ago, but even if you think your manuscript is perfect, it's not. I promise you that. There will be things like SPaG errors and holes littered throughout it that you are just too close to it to see right now.
If you can't afford a professional editor (I can't) then get your editing cap on a year or so after you've written it, distance yourself from it as much as possible then start editing.
It's far from easy to pick holes in your wonderful manuscripts, but the agents and publishers are looking for reasons to reject you and even the smallest mistake will get you rejected and they won't read any further.
Don't give them the opportunity to reject you.
Make them move your first three chapters from the slushpile over the rejection pile and into the "possibly maybe" pile.
I know rejections hurt, but that first full manuscript request will make up for all the pain and tears previous agents and publishers have caused you.
You can so do it, but you have to edit multiple times to get your manuscript to be the best it can possibly be. The longer you wait between the submission and the rejection, the better your manuscript is. I've been waiting for the rejection from two agents since September and October 2018. I would have been rejected within a month or six weeks if I'd sent them a pre-edited version and I got my first ever full manuscript request from a publisher back in August/September 2018 kinda time and I'm still waiting to hear back from them, which is a good thing apparently.
There's no way that they would have made the full manuscript request pre-editing just like all the other agents and publishers I'd sent to, but after at least two major editing sessions, I've got two agents and a publisher possibly interested in my manuscripts. If it can happen for me then it can happen for you too, just make sure you edit every manuscript you send out so that it's perfect and they've got very little reason to reject you.
Get your word processor open and get editing - you can so do this!