Admitting your mistake is the 2nd step in fixing things (finding the mistake is the 1st), and the 3rd step is making sure the mistake doesn't become a problem by fixing he mistake. There are lots of minor steps (which usually change based on the situation), but I've found these are the 3 major steps to get a situation back on track.
We're all human and can rush through even important things. Use this as a learning experience to prevent doing it again.
You shouldn't defend the plagiarism, just admit the mistake and correct it, which it sounds like you are willing to do, if you haven't fixed the issue already.
The things that caused you to make this mistake can all too often be used maliciously as excuses for bad behavior. Don't fall into that trap. Even if they are true, it'll likely be taken as an excuse, rather than a good reason. We've all been affected by changes in our lives due to Covid-19, so there's a little more leeway for things like this getting in the way, sometimes. But that's still not a good reason for making career ending mistakes.
If you haven't already, you should apologize to your advisor for the mistake and reassure them it won't happen again. You can state that it was unintended, but unless you can somehow prove it, like you said you could with the reference in the appendix, then it likely won't reduce the lost trust.
This is a case of "take your lumps" and learn from it. It'll likely take less time and energy than anything else. You "did the crime" so you should "do the time". Because it done was accidentally (intention often matters in law) and you haven't submitted this as a final draft, your crime isn't as serious as it could be. Defending it would make it more serious, so just don't do it. The "time" usually depends on the severity of the crime, so you've lost trust from your advisor, which is a lighter sentence than another Answer mentioned about expelling you. Just work to regain that trust, even though it might not ever be regained, and try to make sure you don't give them another reason for them to lose trust in you.
Martha Stewart is a good example of this. She was convicted of insider trading and, for the most part, did her time and got over the whole ordeal fairly quickly. She could have made a huge deal of trying to save her fame and fortune by putting up a massive fight, but instead she only did what seems to be a perfunctory fight against the charges. She ended up doing her time in prison fairly inconspicuously, then her home confinement without making a big deal of it. She might not be as big of a deal or as much of a "household name" as she could have been without the problem, but she's still around.
There's plenty of people who have made a huge deal about charges against them, only to make things worse. Don't do this to yourself.
As for your feelings about what happened, well, you'll just have to deal with it. Forgive yourself if you can, understand that it was a mistake that you fixed, and over time, it'll feel less like you've done something "horrendous". It could have been worse, but it was caught before that happened. Again, use this as a learning experience. You are allowed those. I know what I'm saying isn't exactly comforting right now, but that'll just take time to dull the edge of what happened. That's normal.
It's actually a good thing you are reacting like this, even though it doesn't seem like it right now. Your feelings will help prevent you from making the same mistake later, due to "never wanting to feel like that again". Just take some time to calm down. In a few years, you'll look back at this as a lesson learned and pretty much everyone else will have forgotten about it.
