She says as soon as she started taking them, she instantly felt better. No more randomly bursting into tears.
But, she is no longer the wife I had. Her conversation is so slow. She regularly forgets what she was talking about, repeats herself. Basic chores are just forgotten about. She drinks more than ever, has started smoking. Her diet is awful. If she can be bothered to eat it is probably a chocolate bar at lunch time. She has gained about 4 stones in weight.
She never took up the counselling that she felt she needed before starting on the SSRIs.
She tried cutting down on her medication a little while ago, but this made her incredibly paranoid. So instead she has had to increase it.
All I see from it is an ever decreasing spiral into ruin.
One of the (unfortunately common) ways people learn that they have bipolar is by being mis-diagnosed with unipolar depression, being prescribed SSRIs, and suddenly finding themselves experiencing 'dysphoric mania'. Not the "everything's great, let's have sex with random strangers" type, but rather the "let's destroy every piece of furniture in the house" type.
But I don't know your situation other than what you've described, and I'm definitely not qualified to tell you what to do. The best I think you can do is to get a second opinion from a competent psychiatrist.
The "let's destroy every piece of furniture in the house" is exactly what happened with one of my parents, it's not a fun thing to witness but they eventually got the bipolar under control with lithium but the toxicity on that stuff is pretty horrible, regular blood tests for the rest of your life horrible.
First off, good advice is found in the mental health hotlines. 1-800-950-NAMI (6264) if you are in the states. They can let you know options that I might not realize.
I was lucky enough that my ex kept his meds up (mostly) and went to the doctor - and that I had an invitation from his psychiatrist to call the hospital if I had any issues. Unfortunately, you don't have this. However, this doesn't preclude you from talking to her doctor. Even if he or she doesn't give you feedback information. He might be willing to change course with her or offer.
You might be able to change tactics and talk to her as well, explaining that you aren't sure this med is doing well for her and about the changes you have seen. She might be willing to go to a psychiatrist with you under the view of saving the relationship.
You also might be able to speak to a psychiatrist or therapist yourself to get some ideas on how to get through to her.
And good luck to you both. I truly understand your sort of struggle, even if I don't understand the specific flavor of it.
The best that a knowledgeable and well intentioned doctor can do is prescribe them in a trial and error fashion, maybe with a tiny bit of guidance from prominence of patient symptoms. But it's largely just prescribe, wait 6 weeks, rinse and repeat. I happen to be one of the 'treatment resistant' types, in that SSRIs don't do jack-squat for me (well, except for when there's a drug-drug interaction, and that's definitely not the 'good kind' of effect).
The medical literature suggests SSRIs are only just barely more effective at treating depression than placebo. There's also an interesting (and in my view, plausible) explanation behind why SSRIs might cause an initial increased suicide risk. A common symptom of depression is 'psychomotor retardation': "a slowing-down of thought and a reduction of physical movements in an individual" (https://en.wikipedia.org/wiki/Psychomotor_retardation). If SSRIs have any positive effect, they occur gradually.
So the hypothesis goes: a patient might be suffering from suicidal depression, but the psychomotor symptoms prevent suicide. When they are prescribed SSRIs, and those SSRIs start having a positive effect, they lift the psychomotor retardation just enough that the patient is finally able to kill themself. It probably sounds a bit strange, but I can attest to how debilitating 'psychomotor retardation' can be.