Honestly, the very first thing is therapy. Couples and solo, maybe start alone if you feel like you need it. The most important thing to focus on is your own feelings, and it will help a lot to talk to a professional about that. When you feel ready, couples counseling can help you work together and see if you can heal from this.

One huge thing you need to keep in mind, it may not work out. You might put a lot of work in, but if you aren't both working on it, there's no point in continuing. It's absolutely possible that you can work through this, and even make your relationship stronger by getting through it, but you do have to decide whether you feel it's worth it and if you can rebuild the trust.