How do i stop being so insecure with my new gf after being cheated on by my ex?

This is the first time i have ever posted in a forum... I am trying to overcome an issue thAt is starting to become an unhealthy part of my relationship...

About 18 months ago i met a girl through work and we started sleeping together. She had told me she didnt have a gf and i believed her. Turns out she did and i was left feeling used and depressed as it took me around 4 months to find this out. I ended up transferring my job to another location as we worked remotly on a small island.

Approx 3 mths later i met a girl at my new location.. Things were good and extremely fun for approx 12 weeks or so till one night she asked me to be her gf... I accepted and was super happy.. 3 days later i caught her in a pool semi naked indulging in a 3 some with a guy and girl...

To which she then told me that she had also slept with numerous other people while she was supposedly with me.. This affected me and i lost my cool. For the first time in my life i self harmed. I went home all dazed and confused and cut my wrist. I didnt want to die i just wanted the pain to stop. And then i spent the next 3 days in hospital in shock at what i had done to myself..

2 weeks after this i met a girl who i slept with. We kept in contact for the remainder of my contract at work (6 weeks) and when i had 6 weeks off over xmas she came home with me and we spent christmas and new years together and she met all of my family... It was a really awesome break together. We now have to be seperated for 12 weeks due to work commitment to which she will then relocate to where my work is because she has good job prospects here also...

We are 3 weeks into our 12 week stint and its getting hard. She told me she has never cheated before and she isnt about to start now. She is a genuine person and i belive this... We talk all the time and are still learning about each other but i cant shake the feeling of being insecure. Its slowly creating a wedge between is and i dont know how to stop being this way..

We have both spoke about our insecurities and promise to talk about everything and we do... But alot of the time she says that she will do something and she doesnt... Or vice versa. It makes me feel uneasy and i have explained this to her but im at a loss as to trying to figure out how to NOT be this way.. I dont want to be like this.

I seen a psychologist 3 times whilst i had time off for anxiety... And depression but still struggling with this. I dont want to push her away..

Any help would be appreciated...