All of us have to decide where to begin when we record. I'm talking about where to begin in our quest to make our recordings better. The number one thing to do for better recordings is to capture a good performance. Get the part right, so that what is recorded is right for the song and the mix. A bad recording is less of a problem then a bad performance. A marginal recording might be fixable. But a bad performance doesn't work with the song, so it can't work in the mix with out a lot of effort...and maybe never!
This single bit of advice will go far toward making your recordings better. Do what you have to do to make the performance a good one. Rehearse. Rehearse some more. Work on the part until it is easy to play, or feels natural. This goes for singing also---especially parts that may be slightly above or below your range. Spend a week singing the part in full voice until it starts to sound right.
If you just can't get the part to sound right, maybe you need to re-work the part, or try something else entirely. Arrangement is a big part of a good sounding mix. You may need to tailor the arrangement to what is possible, rather than what you would like it to be.
Of course, you can always go out and find someone who can do the part the way you originally conceived it. But, we are looking at things here from the perspective of the musician who prefers to do for themselves.
And remember, the arrangement, and the mix are both in service to the song. Most songs can be arranged in many different ways. If you are struggling with parts, maybe it is time for a re-think. Perhaps there is a different direction from which you can approach the song. Change instruments. Bring in the vocals a differnet way. Do whatever makes sense for the song to get over the performance hump.
And simplify---I realize this might not always work, but if you are struggling with performing some of the parts, maybe you should drop them, and see how it works; ---the "more-with-less" approach. Maybe you are used to playing the song with just a guitar. Now that you've added drums, bass, piano, percussion, and background vocals, it is not feeling the same. Try removing some parts. It might be that the song will work with just a simplified version that is more of a "guitar with a few short sections of other instruments" kind of a thing. Try it.
We have the luxury of recording in our project studios without worrying about the big studio clock running. So, experiment and find out what works for you. In fact, your homework assignment is to take a day and try a bunch of things you've not done before. Maybe you can try recording some parts in the bathroom, or hallway, ... or that walk-in closet full of clothes (hint: makes a pretty good sound booth). Try recording a song with just guitar and tambourine.
Maybe those experiments work, maybe they don't. Maybe they give you ideas. Either way, you learn something. And as your knowledge grows, the recordings get better. That's what we are after here.