Hi, Meggy. I'm so sorry you're still struggling so much with this, especially now, of all times. All of our anxieties are already all keyed up as it is, having more about something else, something like pregnancy, no less, on top of them has got to be so rough.
I agree with you: it sounds like an ultrasound probably won't help you here. After all, the tests you have taken and your periods are just as accurate as an ultrasound is. The thoughts you are having clearly aren't rational -- not in the respect you could be pregnant, because you very clearly are not -- so they won't, as you've already experienced, tend to respond much to rational answers.
Since you probably won't be having a new sexual partner for a good while now -- none of us will, after all -- you can at least try and let go of any pressure you feel to get over this to be sexual with others. See if you can let that go, okay? Any pressure you can take off is going to help you out.
At this point, I'd say that you have a few options when it comes to trying to get past this or, at least, to manage it:
1) Putting your focus on everything else, and teaching yourself to stop answering your anxious thoughts. In other words, when you think this way then test or keep going deep into the thoughts, you kind of reward your brain and the thoughts and also validate the irrational idea and the anxiety, so you keep it going that way, even though it feels like you're doing the things that should end it. If you can start trying to answer these feelings and thoughts by NOT answering or validating them, by doing something else entirely when you get them, by focusing on something else.
2) Trying to get out by going through -- have you gone ahead and talked with someone about these fears? Like talking about why it would be so scary to be pregnant, why you think you feel so scared and have so much anxiety about it, but also what you think may have gotten you here? Sometimes that helps. You could do that with a friend, with staff here, and/or with a counselor. I have a could phone lines I could suggest for this if you want.
It might also help to talk about why you think you might WANT to be pregnant. I hear you, you're scared of this, so you probably don't feel like it's something you want. But our brains attach themselves to something like this in part because it in some way is giving us something we want. In this case, that could be a bunch of things, but a few possibilities could be because it's actually easier to think about than something else you might otherwise have to focus on, or because your brain is trying to tell you something you're otherwise just not looking at or dealing with (one common thing I have seen around this is that is seems to happen often to people in bad or abusive relationships, for example).
3) Related to the last bit, if you haven't already, getting qualified help for this anxiety. This is anxiety you are experiencing, and it is a mental health condition that there are quite a few effective treatments for: certain kinds of therapy, some medications, some lifestyle changes, mindfulness work, and a few more. Have you looked into getting treatment?
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead