One thing we know for certain is that as a person becomes increasingly isolated and has no one literally, they can truly confide in and talk to about ANYTHING they wish, their thinking DOES become increasingly narrow, myopic, and a bit skewed relative to reality; this situation does incubate uncertainty and anxiety
about how to interpret the actions of others and thus, helps incubate some paranoia. A famous psychiatrist name Sullivan was one of the first to emphasize the great importance of having a very strong and intimate connection with someone to share one's personal thoughts with because social feedback from a confidant and receiving an objective reaction to what one is thinking tends to help correct that thinking.
So it is important to have a confidant confirm and more importantly stretch the range of our hypotheses about events such as why your girlfriend did what she did on a certain day. Sullivan called the effect of this interchange of one's personal thoughts with a confidant, 'corrective emotional experiences'. Having someone to really talk to keeps our thinking from heading off on a dirt road to 'who-knows-where?' , instead of staying on the main highway.
My main reaction right now is to suggest that you really do need to find someone you are willing to trust completely and take this 'risk' with to share what you are perceiving about things---the mind games, the social manipulations of this last woman you dated. You pulled away from this friend at work and this is exactly the thing one does not want to be doing, because you need intimate conversation and the chance to share your ideas---you need to hear yourself talk out loud about this stuff and get feedback from someone you trust. If you can't do this with a friend, then I'd suggest you find an objective, dispassionate, very smart alternative person, a clinical or counseling psychologist for example. I'm certainly NOT suggesting anything is wrong with you in terms of your mental health but this is one 'mechanism' that would substitute for a very close, trusted friend if you don't think you have one available to you right now. And I'd get on this right away. What do you think?