Stop anxius over thinking in a relationship.

It's so hard to put into words what actually causes my anxiety and what my long standing fears are. They are always there in the back of my mind, but are so much more profound, to the point of debilitating when I have a boyfriend.
I describe myself as quite a needy/clingy person and
If I Don't get the affection and attention that I crave every day (which obviously i don't because The rationale part of me knows that's not natural) then my mind goes
Into over drive, saying things such as:
"he hasn't text you because he doesn't like you anymore"
"He is probably with another girl"
"when a better girl comes along he will dump me and go off with her"
"if he's busy or cancels a date etc, it's because he doesn't want to see me".

It's very ironic that i also know that the main thing that will ultimately drive him away is me thinking like this and living in constant fear that these things are true so I know it's a self fulfilling prophecy really coz my fears will inevitably come true if I keep this up because nobody should have to put up with being accused of things which aren't even true as he has never even given any inclination that they could be. I do know deep down that at the end of the day, the main reason I will lose him is if I carry on like this and that is something that I definitely do not want to happen. Despite this, I still can't seem to stop these thoughts. They are very powerful and just pop up when I least expect it. I try as hard as I can to rationalise things and say
" no, I know these aren't true etc etc"
but it's as if it goes in one ear and out the other and doesn't make the slightest difference to the thoughts. They just keep whirling around my head at top speed and I am unable to suppress them.
Some of my friends think that due to this being about being in a relationship, then I should be single. This makes sense but at the end of the day it isn't what I want and I don't want to let my head feel like it has won by not letting me be in a relationship due to these thoughts.
I have been with my current boyfriend for 3 months and apart from my crazy thinking everything is great. And he is very lovely.

Outside of a relationship I am actually I pretty confident, outgoing person.
I have had some bad experiences of men, one boyfriend I dated for 6 months had 3 other girlfriends at the same time as me and several have cheated before too. Also with all the
Coverage of cheating and people
Being unfaithful around in newspapers etc, just makes my already non-trusting mind of men even worse.
Please can anyone offer me any advice?

Hey, nobody's perfect. You have a tremendous insight into your own problem, which is a really good beginning. It is basically called insecurity. But you seem to know these thoughts are irrational, and you are taking active steps to talk yourself out of them when you become aware of them. You know they are making you unhappy and threatening your relationships. The only thing I might suggest, is to tell your boyfriend you struggle with these issues from time to time, and that he could help by reassuring you when you feel that way. It is not unreasonable to ask for his help. If he cares about you, he should be willing to do this. You might have to figure out the best way he can communicate his reassurance to you, and tell him exactly what you need him to do so there is no resentment or ambiguity. But don't make a big deal out of it, or nag him too much about it. People in a relationship should enjoy each other's company, and when that is missing, it stops being a fun, positive experience and becomes just another burden in life. Good luck to you.