I'm scared my anxiety will ruin my relationship

Hi.. So.. my anxiety has always affected my love life.. I'm only 19 years old and I am now in my first relationship, we have been together for only 5 months but have a past together. He was the first person I ever fell for, I never let myself with anyone else and when we first met 15 months ago things went terribly wrong.. I was too insecure and he had too much of an ego so things didn't work out, and some terrible things happened.
I have always had incredibly low self esteem, however over the past year or so it hasn't been as bad as it used to be..

Despite having several people say they were interested I always thought they were just out to hurt me, laugh at me, make a fool out of me etc. I had a very severe anxiety attack last April when he (my now boyfriend.. Let's say "Chris") in sorts, rejected me. And it took me a long time to get over it, lots of anxiety attacks later I met someone else who I tried to move on with.. It didn't work. Weeks later me and "Chris" started talking again and I ignored the negative thoughts and just went for it and I am so happy, we are so in love.

However every now and again (usually a few days at a time every couple of weeks or so) I'll shut off from him and my anxiety will be through the roof.. I'll constantly be thinking of him with someone else and I'm scared it's going to ruin our relationship. We have booked a few trips and over the next few years plan to travel as many places in the world as possible, and we've even spoken about marriage and children (two things I were completely against before this relationship) so it definitely is the real deal.

I guess I'm hoping for advice on how to try and, not so much switch off, but at least something to relax my mind when these thoughts go through my mind.
Just Fyi, my anxiety is usually fuelled by rejection and failure, and I am very socially awkward, so any major- or even minor for that matter- social events (something as silly as sitting downstairs on a double decker bus) can kick off my anxiety. Any advice or even just your stories would be amazingly appreciated.. This is the first time I've reached out for help and I already feel slightly more relaxed.

Thank you☺️

Just to add to this I have always been right when I've had doubts when seeing other people, and always proved it. Also just recently we went through a bad few days were some of my doubts in this relationship were proved right.. Nothing since we have been together, only from before we were together.. So no unfaithfulness.. Just a few lies in the weeks before getting together.. So obviously this has made my anxiety and paranoia that bit worse.