Oh Worry this sounds very distressing and exhausting for you, I really sympathise. Firstly, I want to offer a tiny reassurance that you probably have done nothing to harm your baby. Unless you were doing coked-up black-out boozy weekends or sharing your heroin needles throughout the first trimester, you're really unlikely to have harmed your baby. There is so much information out there to terrify us, it's no wonder we get anxious, but the chances are really very slim.
As for toxoplasmosis, here in France it is standard to test for immunity, and I know I don't have it (but many women I know do/did and so carried on eating whatever they liked throughout pregnancy!). But my midwives have been very clear that the only thing to do is make sure I wash raw vegetables well and, if we had a cat, not change the litter tray. It's really that simple. So I soak my salad veggies in water & vinegar for a few minutes before rinsing off and preparing the salad as usual, and that's it.
On the baby shower, I think you need to be honest with your mum & sis. Yes, they are excited, but this pregnancy isn't about them, it's about you and your baby. If you're struggling with small steps, then maybe a 4 hour journey and a cat house and lots of people is just too much for you. Are you able to talk to your sister about your anxiety? Might there be things she could do to reassure you (ie cat out for the whole day, do a quick spring clean before you arrive?). If you can't easily discuss this with her (I know my sister might get a bit arsey if she thought I was suggesting her house wasn't clean enough!), then simply explain that your anxiety is such that you can't go. Perhaps you could ask them to do something later in the year, after the baby has arrived? No plans yet, just play it by ear once the baby arrives.
Please look after yourself and don't be too hard on yourself. Your body and mind is going through so much at the moment, and you're clearly doing your best. Take care