Inability to let things go

First off, that physical sensation you're describing sounds a lot like anxiety. However, said physical symptoms can be manifestations of much more serious problems.
Case(s) in point: Two of my friends have claimed such symptoms in the past few years. In both cases, these symptoms were written off (by medical "professionals" and others) as anxiety attacks. In both cases, they were symptoms of heart conditions. I'm sure this isn't what you want to hear (the thought of heart problems could simply lead to more anxiety), however the responsible course of action, in my opinion, would be to have yourself checked out, just to be safe.

As for the trust issues, well, those aren't something you just "get over." My recommendation would be to openly discuss the situation with your girl (if you haven't already). Realize that while all women are fundamentally similar in the sense that they are all capable of driving us to completely illogical states, they are not all identical. She is not your ex. The hard truth is, if you aren't able to trust her, you probably aren't emotionally prepared to be in a serious relationship.

Now, as an example of my own emotional maturity, I'm going to go into the wastes, shoot some super mutants in their grossly deformed heads, then go to bed...

...perhaps I shouldn't be giving advice.