Sound Engineer's Hard Work [HD]  (LOL!!!)
 
https://youtu.be/G2Rhh_4GZmU

If I'm doing session work, I make sure I'm on top of my game and ready to improvise. Having a good handle on theory is pretty important so you have a "map" if you need to wander off the path.

Most of the sessions I've done have been pretty laid back and collaborative but I do remember one I did about 15 years ago where the engineer/producer/writer had played the guide on keyboard and wanted me to redo it on guitar. Easy enough except after every half a take he would stop and nitpick any tiny thing that was slightly different to his guide, even during the time I was feeling my way around the piece (this was a "hear it for the first time 5 minutes before we do it" session).

In the end I had to tell him to just let me play thorough a couple of times alone and then realise that exactly emulating his scratch keyboard tracks is kind of missing the point of getting a live guitarist in.

We got there in the end but man, that was a frustrating session! So let me add that apart from having your chops up and knowing your theory, a good wad of patience and an open mind (on both sides) will go a lonnnng way!

Davedud it sounds like we might be similar in some ways. You seem to be moving about wearing many different hats in trying to achieve a goal. This can be a real problem with the instrumentalist/studio engineer.
 
We end up spreading ourselves too thin if try to do it all, so we usually need to pull off of something to make room for improvement in another area. In the end your skill set should eventually level off for what you want to do if you devote enough time and energy into the right areas. In some cases you might just need to ask for help in tracking something you can be proud of.
 
To add insult to injury many of us here are learning a new daw. Not something we had planned to do.
 
I know musicians who can play rings around most common men, but they know almost nothing about recording. I know others who are excellent at tracking and mixing but they can hardly play a lick. I'll admit I have a bit more respect for the first group. You can always find someone to record you, but it's tough to find good dedicated musicians.
 
It all begins with sizing yourself up and being real about your limitations and goals. Push yourself a bit. Don't settle for a low goal. 
 
I play multiple instruments but not all well. I have a wall full of guitars but I don't really consider myself a guitarist. Yeah I can play, but playing guitar well is usually a lifelong dedicated pursuit which I don't have time for. I play plenty well enough for the simple gig or for tracking. That still doesn't make me a "guitarist". I am just a hair over 2 years into learning violin. I can play over 50 songs by memory. I'm sharpening my techniques every day. Put me in the studio though and I pull my hair out trying to get  perfect violin takes. Maybe after 4-5 years I'll be there. Put me in a bar with drunk Irish men and I can play plenty well. Compare me to anyone who had extensive classical training and I'm a joke. Same goes for harmonica and bass guitar which does take skill no matter what you might have been told.
 
The only "ist" I'm really good at is piano and keys. I can lay a solid melody along with chords and singing sometimes in one easy pass. I can meander the heck out of a bouzouki too. lol.
 
So pick one or two things you want to be best at because you can't do it all. JMHL YMMV.

These studio session musicians made history by making bands sound better in the studio.
 
The Wrecking Crew
https://en.wikipedia.org/wiki/The_Wrecking_Crew_(music)
 

The Wrecking Crew was a loose collective of session musicians based in Los Angeles whose services were employed for thousands of studio recordings in the 1960s and early 1970s...

...The Wrecking Crew's contributions on so many hit recordings of the era went largely unnoticed until...

 
There is also a very interesting documentary film about this crew.  A sound engineer buddy of mine pointed me to this film a while back.  Very enlightening! 
 
http://www.imdb.com/title/tt1185418/