How to deal with insecurity in a healthy way?

I made a post earlier - quickly realized it was ridiculous and none of the backstory matters. It was up for less than an hour before I deleted it so hopefully no one saw the awful rough draft.

The black and white facts:

1. I am dealing with past trauma that has my sensitivity level on high. Things that were not previously an issue are now becoming an issue to me.

2. I am in the most secure and safe relationship with someone I am madly in love with. I suddenly and feeling insecure because of his relationship with his ex girlfriend who he lives with.

3. I am positive there is no sexual nature happening with them.

My insecurities seem to be triggered when:

1. I see them chatting multiple times throughout the day while we are together. I noticed what feels like an increase in their communication.

2. When he mentions her I get sad thinking and hearing about the things they do or have done and how coupley it all feels. He seems to be bringing her up in conversation more recently and I think this is happening more now also because of the higher frequency.

3. He recently mentioned that she has been playing a video game he introduced to me. It’s a really special game with a lot of meaning. He told me he doesn’t share it with many people. It hurt me to hear she was playing. Knowing how much we bonded over it and how excited and hype he gets about it. It felt…really hard to hear that.

4. When I know he’s home with her I have intrusive thoughts setting in where I imagine them cozy and cuddly watching a movie and it makes me really freaking sad.

Nothing he’s done since we’ve started dating has ever made me feel insecure before I started going through the things I am with dealing with my traumas. I just don’t know what to do here or how to face this and feel good again.