Not So Fast,
May I suggest a fairly simple answer to your question, which I think will quickly solve your little mind problem here?
My suggested solution will cost you nothing, just your thinking very thoroughly about one of your own statements.
This was your question: "...Sometimes I just can't fucking relax when its time to fucking relax! How do I turn the big head off and have a good time?"
Your statement written above your question, the one which I am suggesting you think very carefully about, was this: "I can't stop thinking about shit that you should not be thinking about in the presence of a wonderful woman who wants to please you."
*Why* shouldn't you be thinking about "shit" (meaning your life problems) when you are with a beautiful woman?
If you analyze it, your statement contains a hidden assumption: that sex should not ever be mentally associated with problems.
But (let me argue to the contrary) sex is, very often, definitely mentally associated with one's problems - as a temporary solution or relief to them!
So that you can see what I'm trying to say here visually: go to www.beautifulagony.com That site has hundreds of pictures of the faces (only) of women and men (mostly women) having orgasms. I'm new to that website, which someone recently suggested on a TER Board. There are currently two free 4-6 minute videos, of a woman and one of a man. (You do *not* need to sign up, or give your credit card #, to see what I'm talking about.)
I would suggest to you that the people enjoying sex there were not hampered, as you apparently are, by the disabling thought that sex must be separated from life's anxieties. On the contrary, I think most of people seen there, and men and women generally, are enjoying sex as a little *relief* to the daily anxieties they are/were already feeling.
If you keep trying, as you are, to *separate* sex from your normal, analytical, mathematically able, though worried, mind - that's *never* going to work. And you'll keep getting more and more anxious about trying to get your mind to conform to what you feel is 'normal'.
So: try this little fun exercise. When, in the next few days, you feel like masturbating, go right ahead and do so, just as you normally would. But, as a personal experiment, instead of thinking about some 'wonderful woman' then as you normally do...try to think instead about all your problems...right at the very same time that you're feeling sexy!
That way, you'll quite quickly begin to associate sex with what it's meant to be biologically: *not* disassociated from your life problems, but, to the contary, as a relief to them.
Next exercise: the next time you're with your favorite trusted Provider, talk with her first, and tell her a little about your experimentation. Tell her you'd like her to give you a BBBJ (or whatever else you really like) at the very same time that you are telling her out loud about some life problems you're having. (Since she's a paid professional, she'll do that. This is one of the advantages of our hobby. You can have someone wonderful do what your SO or wife might not.)
I guarantee that you will find, as you're thinking about your problems, even saying them out loud, your "little head" will take over your "big head". Sex (as you'll see clearly on the website) is a beautiful build-up of tension...followed by release of that tension. So let sex be associated with your life's tensions. That's not something to be avoided, but perfectly OK.
I've been through a long and expensive psychoanalysis. You can try that route if you like. But why not try first what I'm suggesting here. It's a lot cheaper. You have nothing to lose. I'm 100% confident it will work for you.
Let me know any questions. (LG, by all means, let us know if you disagree with me.) And let us know, please, if it works for you.
In summary: you don't have to force your mind to remove your tense problems to enjoy sex. On the contrary, sex is a natural resolution to tension. The more tense you are, the better sex can make you feel. It's nature's free psychoanalysis.
You may want to reflect also on the sociological pressures that led you to feel you have to purify your mind and feel 'relaxed' before you can enjoy sex. That's a lot of malarky, in my opionion - currently popularized, but wrong.
Sex is the best drug there is. It's meant to be enjoyed, whether you're feeling relaxed or worried. Don't try to fight whatever you're feeling. Just feel and enjoy, at the very same time.
All the best,
BG