I have seen in my own family a very unwell aunt when her mental health spirals, she will be hospitalized and her medications are reviewed, after a few months everything balances out for her and she starts feeling well and engaging in life again. At this point she feels she is doing really well and no longer really needs meds, and starts taking them erratically then eventually stopping them completely, she becomes suicidal again, she goes missing, we are all frantic, she is hospitalized and her meds rechecked and the entire cycle starts again.
There is nothing we can do, we cannot force her to take medication, we cannot reason with her when she stops taking them, when she restarts them she feels great and becomes convinced she no longer needs them as she is "well". Her adult daughter is heartbroken and trying to cope with her mother being in this cycle, her daughter has a demanding job, is married with children of her own and her mother cannot be trusted alone with the grandchildren. There is no respite for her adult daughter and she is always waiting for the phone to ring that her mother has gone missing again but this time is not coming back. Her daughter has been married twelve years but never had a honeymoon or gone on holiday as her mother cannot be left. This has been the cycle for decades, my aunt is in her seventies now and it is still ongoing. This is the reality of people in your family being mentally unwell, it can break the family members that are there trying to support them.
I hope Britney has support I really do, but there is only so much people around her can do, she is clearly unwell but I don't know who can reach her now.