It certainly sounds like some kind of anxiety disorder; being paranoid about relationships, worrying if you said the wrong thing, or didn't say enough, or said it too soon, or left it too late.. usually coupled with very low-self esteem.
A lot of which I experience too, to varying degrees in different ways.
DO seek professional help. The inconvenience of getting help in your location is dwarfed by the impact your anxiety is having on your life every single day. Medication can do a lot to take the edge off any immediate physical reaction you may have to anxious situations (any of: palpitations, perspiring, voice breaking up, nausea); which makes it easier to tackle the issues in your mind. CBT therapy can be very effective in making you look at these situations more objectively and changing how you view and react to them.
It's probably a very common disorder; and can affect people in different ways.
I suffer from it too. I'm a single guy, 36; and was recently in a bar talking with a friend when I was approached by an attractive girl. I immediately froze up, couldn't think of a thing to say. After an awkward few moments, she walked away, embarrassed. I was able to just resume the conversation where we left off, but felt awful at how I'd made that girl feel. 20 minutes later, I was approached by another, same deal. Later in the night, a very attractive girl kept coming over to dance right next to me; we moved to a different area and she came right over to us again, but never in a month of Sundays could I approach her.
The weird thing is, had we met in different circumstances - at work, in a shop etc. - I'd probably have had any of them laughing until they wee'd a little. But in bar/club, not a word.
On other occasions, it's very much like you experience, double & triple thinking every thing I do, or am about to do, or should have done, every message I send, or don't send..
Hang in there, you're not alone.
