Hey @Birty and welcome to the Forums, we are glad to have you!
It sounds like you are in a really tough position here and I want to acknowledge how much of a toll that can take. I hope that I can outline a few suggestions here.
Talking with your partner
It can be worthwhile to dedicate a time to explain exactly what your concerns are and what you are feeling.
If your partner is insistent on stopping their medication, it can be helpful to talk about a support plan which can help you both have a set plan of what to do if they become unwell again, and what types of symptoms might indicate this.
Talking with a GP
Another option is booking an appointment to attend their regular GP to discuss your concerns together, as well the reasons your partner has for wanting to stop medication. If your partner does not like how the medication is making them feel, it can still be an option to trial a different medication or dosage depending on what the doctor recommends.
It is also important that your partner is medically supervised should they stop their medication and that you are both aware of any withdrawal effects to look out for.
Discussing other options
If your partner is not willing to continue with medication, what other supports might they find helpful? There is a whole range of supports available, from group supports, peer supports, to more traditional counselling and psychotherapy.
Can I force them to take medication?
The short answer to this one is 'no'. What you can do is encourage them and outline your concerns. However, if you notice behaviours that are dangerous to your partner or to those around them, you are able to call your local Crisis Assessment and Treatment Team (CATT) for immediate support and assessment if required.
SANE has some more information about talking with someone about your concerns and when to seek help here.
I also recommend the SANE Support Centre for a further discussion.
This is such a challenging situation to be in and I hope that this is able to provide a little bit of clarity. Take good care of yourself 💛