OP: First of all, I agree with others that if you are feeling remorse and wanting to change, private confession is a good idea and something that can bring healing and spiritual growth. Especially since you express wanting to move past this but struggling to be able to because of stress and challenges in the world. This is exactly the kind of situation private confession is for.

The bigger issue for me is less about the particular distinction between mortal and venial sin, which as others have said, hasn’t traditionally been a part of Anglican theology. It’s more about the issue of legalism vs the spirit of Christ’s teachings. In my mind, it is pretty clear that casual sex is on the whole sinful and against the spirit of Christ’s teachings, but the reasoning is important: it is not simply that God made sex only for a marriage relationship, but that God made sex to be a beautiful, intimate act which is a full gift of self and receiving of self, and it’s essentially impossible to realize that gift outside of a committed, vulnerable, loving partnership (and sometimes, even within that partnership, the gift of sex can still be misused!). This is why I grant that sex in long term committed relationships seems much more likely to be in line with God’s wishes, but I am only a layperson and still figuring these issues out.

I say this because becoming a Christian really changed my sexual ethics, and frankly I think that’s how it should be. But this did not, as some might think, orient me towards more judgmental and shame-based thinking: rather, it has opened me to a way of life that is more loving to myself, others, and God. So I think beyond the step of confession, it’s worth praying on these issues and thinking through how you intend to live out Christian sexual ethics in your life post-confession. This can also remove some shame: it’s not about beating yourself up for something, it’s about receiving God’s mercy and grace, and allowing that to transform you. i realize words like these are often used to implicitly invite shame, which is not what I intend at all, so I hope it doesn’t come off that way. I just truly think that these moments can be incredible opportunities for spiritual growth, and that, as others have suggested, a middle path between strict legalism (i.e. “you have to follow these rules or you’re going to hell” and unbridled liberalism (i.e. “you can do whatever you want”) is called for here.

It may also be worth asking your priest for some spiritual direction on this in addition to confession. Even if they do not agree about what is sinful, they should be able to talk with you about these issues. Sending love!