It sounds like you've had a really rough time with medication, I'm sorry it's been such a difficult road for you.
From what I have been learning (as part of my Psychology Masters), it is unfortunately quite common to have significant side effects when adjusting to a new medication / tapering off a previous medication. I'm guessing the combination of doing both at the same time is likely to produce even more debilitating side effects than just doing one or the other?
I'm sure five weeks feels like an incredibly long time to wait for your next appointment, but I would definitely recommend NOT doing anything to reduce or adjust the medication dosage you've been prescribed until that appointment! Medications need a certain amount of time to stabilise in your system before the effects may be seen. You mentioned that this is the last line of medications left for you to try so I really think you need to give it a real chance?
You say you're (understandably) really anxious about the whole process and I wonder if perhaps there are some things you could try to reduce that anxiety in the meantime? I'm not sure if you've been seeing a Psychologist alongside your medication for your Bipolar, but sometimes it's helpful to try and challenge any unhelpful thoughts we're having through CBT (Cognitive Behavioural Therapy). We know that our thoughts can have an impact on the way we feel, and then how we behave: So for example I might have a thought, "This medication is really not working for me, it's useless to keep trying"; then feel even worse about the medication as a result, so stop taking it. A more helpful way to think might be something like, "This medication is really not working for me, but I need to give it a bit more time as the side effects may ease soon"; as a result I hopefully won't feel quite so negative about the meds, and keep taking them. Sometimes just a small shift in thinking can really help and it might be worth giving this a go?
If this isn't quite working for you, strategies to try and relax a bit more, like Yoga; or Mindfulness, (I personally like the Smiling Mind app) might also be worth a try?
I hope you're able to keep persisting! If you're not seeing a Psychologist to work through some other ways to manage your Bipolar I would also recommend looking into that, as there's a lot of evidence for the effectiveness of strategies like CBT in stabilising mood in Bipolar Disorder.
All the best, SammyD