Sounds like it could be one of a few things. Have you been cheated on before, or someone close to you (parent, sibling, best friend) cheated on by a long-term partner? If so, it makes sense that you've got the mindset that it happens and you need to be scared. There's also what's referred to as 'relationship OCD' - obsessive thoughts/compulsive behaviours connected to your relationships. It's also possibly that your self-esteem isn't good and your insecurities make you think he will cheat.
Whatever the reason, though, the only thing that is currently destroying your relationship is you.
Can you imagine how he feels, hearing that you don't trust him, over and over again? That you think so little of him (in his eyes) that you think he'd do that? That after a decade, you still can't let this idea go?
You have to get help. If you can afford private counselling, please get that. Go and see your GP anyway - if you can't afford private counselling, they can refer you for CBT (good for very specific negative thought patterns) and potentially try prescribing you a low dose of an SSRI to help with the anxiety/obsessive thoughts.
Try looking up resources online for anxiety/obsessive thoughts - this website (just Googled, so can't vouch for it directly) for example has worksheet downloads, mp3 files to listen to, and plenty of info about how to deal with unwanted thoughts www.getselfhelp.co.uk/self-help/.
There are also online CBT courses, if private therapy isn't an option, which will be much cheaper. You are either: worrying excessively (aka anxiety) OR having obsessive thoughts.
CBT and help for anxiety are aimed at reducing those negative thought patterns, and there is plenty of information out there to help.
Seek it out and use it - it is miserable to be trapped in a brain that won't let you know peace because you need to be on high alert for the next 'danger', and it is unfair to your partner to make him continue to shoulder it, too.