Hi Ged,
I'm no 'pro' pro in fact I'm not even an ME I'm and producer and mix engineer but I really empathise with you on this as I go through the same thing all the time.
What I do is be really hard on myself.
The fact is this guy is coming to check out your work and it's make or break time!!
You have two options,
a) tell him you can't do it and he's wasting his time
or
b) get the job done
No one on this forum pro, amateur or anything in between can really help you (but you already know that!)
I presume this isn't the first time you've had this feeling? What happened the last?
Did you get through it? I thought so.
The bottom line is, if you really want something you can get it.
You've already said you can do the job, so there's no real lack of self confidence. Is it actually more fear of rejection. Are you afraid to put in 100% in case this guy doesn't like it? At least then you'll have an excuse, 'well it wasn't my best'.
You'll do the job. He'll probably really like the work and you'll be kicking yourself as you listen back with him because those few hours you spent on the net looking for sympathy could have fixed those flaws that are embarrassingly leaping from the mix ten times more apparent now that he is there.
If you know there are problems, fix them now because if you don't the next ME that the guy goes to see will have.
What's the point telling us that there's a problem with the kick drum, you know it sounds wrong and you know what it should sound like so get it there. If it means putting it through every bit of gear you have in every combination till you find the solution then do it.
I've been there so many times, kicking myself because I know i could have done a better job and trust me, it hurts a lot more then the pain of doing the work.
In fact it's only in the last six months that I've beaten myself into putting in 100% all the time (well most of the time
) I tell myself to get over it and I pick the toughest part of the job, the bit I've been dreading the most (probably the kick drum in your case?) and wildly attack it with the attitude that 'this fckin kick drum is not going to beat me' I reference a sound that I want (usually in a mixing situation btw) and I'll stay at until I better that sound.
It all basically boils down to the difference between a pro and a semi pro. The semi pro will settle for second best. The pro will strive for perfection. He may never reach it but you can be sure he'll get some good results along the way.