First of all, let's call a spade a spade. You're not just clingy. You're pathologically jealous. And if you don't get it under control, you risk making your worst fears come true and driving the guy you love straight into the arms of another woman. However, you're to be congratulated on the fact that you have the maturity to realize that the lack of trust is a problem. You've made the first step, which is realizing that there is a problem here that you need to deal with. well done-that takes guts!
The first thing you need to do is to realize that mistrust and jealousy, if allowed to grow and fester, will destroy your relationship far more certainly than the off-chance that your boyfriend *might* cheat. This is a big and dangerous problem for your relationship. Therefore, it's of paramount importance that you try to conquer your feelings of insecurity, and endeavour to get to the bottom of why you feel this way. Have you had bad experiences in the past with fidelity? Or bad role-models? Why is marriage so important to you that you can't just sit back and enjoy what you do have - a lovely relationship with a good and patient man? Think carefully about the patterns of thought and behaviour that have led you to this point. By understanding them, you can conquer them and start to move on.
Then you need to work every day to improve the way you feel. First of all, draw a line. Tell yourself that you are going to stop checking his texts and his email. It's NOT healthy and it's an invasion of his privacy, destroying any chance of trust between you. Secondly, try to correct your jealous feelings. I have struggled with feelings of insecurity and jealousy myself in the past, so I know how strong they can be, even when you KNOW for a fact that they're irrational. One thing that helps me is to get a piece of paper On one side, I write down all the crazy fears I have - without diluting them. Then on the other side, I write down a sane, rational version that is in proportion to the situation. So, for instance, on one side I might put 'Boyfriend arranged for us to go out with his ex for dinner - oh no! What if he still loves her?' and on the other side I'll write 'He said they argued all the time and couldn't get along at all. And anyway, they dated ages ago! And he constantly says how much he loves me'. You have to keep persisting with it when you feel insecure, but in the end, the process of 'correcting' the thought that's out of proportion becomes automatic, and you will hopefully find more security and peace.