One of the most difficult times for me, personally, on any project is the middle section. You have some stuff done and it all seems like it's fitting into place but you know there is still a long road ahead until it's finished and often you don't see how to get there clearly. I find this time really difficult and I have a habit of comparing the current state of a project to finished examples. Most of the time this comparison is not exactly favourable on the unfinished work.
For me this whole stage of the project process is a little bit like the infamous scientific proof where one section simply reads "magic happens here". That "magic" is this middle part of the project or more specifically it's the transition between the middle section and some beautiful end state. At some point it stops being a middle of a project and becomes the beginning of the end. The home stretch, the last lap.
My main issue with this process is mental, it's the mental malaise that comes with comparing to finished work and not knowing how to get the current project to that state. Even if I do know how to get there, that can still be difficult because it doesn't feel like it's possible to get to that end state. I can genuinely start to feel depressed about the fact that I don't seem to be any closer and can't get past that middle ground and over the "magic" bridge into a clear path to the goal. Recently though there are two techniques that I've been using to combat this.
The first technique is something that I call current-state-mindfulness. This is the mental act of trying to take delight in the current stage of the process. If I am recording guitars then I am trying to really enjoy recording them, geek out and get really into it. Find the joy in mic placement, amp selection and all the other stuff that goes along with it. To quote Ze Frank "Try not to view the current state as just a stepping stone to something else, and if it is then try to become fascinated by the shape of the stone".
My second technique is cathartic side projects. These are basically projects that aren't what I would consider core to my main goal or are styles that I don't really particularly have anything to add to but they make me happy to create. Recently i've been working on a serious of metal tracks under an EP called Adrenaline Fuel. These have no vocals and I set up the basic mix of these in a pre-production stage before I started. The goal of them is to enjoy making them but also to allow me to have something musical that has a quick turnaround. I don't spend days agonising over the mix or this sound or that, I just make them some good-ish and then release them. The point is that finishing them gets me through that mid-track malaise. I often use them as a method of getting a break from something that is much more involved or mentally taxing. They also don't sound bad at all which is a nice aside. I often listen back to them just for the fun of it. It's almost free-ing to listen to something i've made and not criticise it.