Dude,
That's cool that you are realizing that you cannot do this alone and that you are reaching out? Not to offend, and you do not have to answer this online, but do you totally trust yourself. I have noticed in many relationships that a person who is dealing with what you are, deep down is feeling that way because they do not trust themselves not to cheat and they project that onto their mate.
I mean no disrespect by the question. But dig deep within yourself and see if perhaps you think you may cheat, adn therefore if you would, she may be willing to do the same.
Your post spoke to a 7 year relationship I had a few years back. Overall, it was intensely miserable and being paranoid as you put it drove our relationship into the ground. However, you are seeking advice and I am sure things will be much better for you my friend. Do keep us posted!