Murdoch, since you say it's your thinking that is causing this, have you seen a therapist at all to see why you come up with such thoughts? If you can disassemble the trigger, the thoughts might not be able to get you. I don't think just willing yourself not to think them will help.

Trust issues are difficult and nearly impossible to work with alone? And there's no way to really figure out if it's because you've been or seen someone betrayed or if you're just afraid of the intimacy and wrecking things ahead of time, etc. I'd find a counsellor to talk with and see if you can't get some sort of handle on it all.