Found messages looking to meet other people on partners phone

I (M30) found messages on my partners (M32) phone looking to meet other people for sex.

My partner and I have been somewhat involved since around 2018, but it was casual in the beginning and we’d frequently date other people.

Around 2 years ago we made it serious and became a couple. In the beginning, we started off open with very struct rules of only meeting other people together, and people we didn’t know, however, in the end we both were messaging (together) people we knew.

I started feeling insecure at this point and had a deep conversation with my partner, and we both decided to go monogamous. I was relieved as while seeing other people together was fun, I had a wave of anxiety lift completely from me and we decided to just be us.

Now, over a year later, I’ve just been on my partner’s phone and he’s on websites looking to meet other people for sex. He doesn’t know I know yet.

We’ve been living together 8x years at this point and I don’t know what to do. I’ve not confirmed whether he’s actually met anyone, but he’s definitely been looking.

The real sting is that I’ve been thinking of proposing for the last few months as we’ve been really happy for a while.

We also have a cat, and we’re going on holiday in 2x weeks and I don’t know what to do. I don’t want to end the relationship, but I don’t know how I can trust him and move past this.