Bringing up past hurts that haven't healed is always a tricky thing to do in a relationship, for all parties involved. They're likely to feel hurt/concerned/guilty over the past transgression and you're obviously still hurting and probably having some trust issues from it as well.
Infidelities are just kind of like that... I hate to say it, but it's one of those things that are really very hard to ever fully recover from. Regardless of the reasons surrounding the infidelity, it's a hurt that damages the foundational trust of a relationship.
But, to ever really move forward, you two will pretty definitely have to be willing to talk openly and honestly about it, as often and as many times as it takes for the both of you to completely heal. This requires a willingness to be vulnerable with each other at a very deep level. Exposing those bits of yourselves that allow you to heal, also means exposing those bits that cause you to hurt, because they're the same bits. That's not easy to do.
I know it's cliche, but "I feel .... " statements can help here. It's not going to be pleasant, or easy, to admit to your partner that you feel afraid or insecure in the relationship, but it kinda sounds like that's the truth. When you feel that way, your partner kinda needs to know, if you expect them to respond to it and work through those feelings with you. Likewise, when they feel guilt, shame, or whatever, you'll need to know about that, in order to work through it with them.
In my experience... One of the worst parts of being in your shoes is how it tends to unbalance the relationship. The person who cheated will likely be excessively apologetic, and likely to become something of a doormat afraid to voice needs because they're afraid of causing more damage, or seemingly not apologetic enough, trying to act as if the infidelity hadn't ever happened. And sometimes, they'll teater back and forth between the two. This imbalance will likely rear it's head for you as well. At times you'll likely be super understanding and forgiving, and at other times, well, not so much.
This can rear it's head years after an infidelity, long after you both believe you've moved on... A simple little tiff over something trivial will push those same buttons and there that raw nerve will be once again...
I mention the imbalance and the way that the emotions you're feeling now may resurface later because it's important to remember that forgiveness is an ongoing process. It takes work. Those "I feel .... " conversations are almost never a one and done proposition. Be prepared for that.
I know it wasn't a part of your question, but this comment hit really close to home for me:
in our case it had to do with a lack of communication revolving around my partner questioning their gender identity (which I didn't know about at all) and whether or not my sexual identity would still "allow" for them as a partner. Add that in with an available ex-partner of theirs and it was a recipe for disaster. –
I've been in pretty much exactly this situation. I know, I'm probably overstepping here, but in my case there was a lot more to it, that came out over time with those "I feel .... " conversations.
Single causation is rarely accurate when it comes to these situations. I won't tell you what to do or how to feel about it, just know that while there may be reasons, reasons aren't really excuses. As in, no matter how good, or awful, the reason may be, it's still going to hurt, and you're still going to have to work hard to get past it.
It may be tempting to try to accept the excuse and force yourself to "forgive and forget", without having all of those really hard "I feel ...." conversations, but it's not something that I think can be forced, even when you really really want to.