Is this daily mood swings or solid depression or mania? Has she been diagnosed? If it is daily she could be rapid cycling, usually accompanied by an unreasonable amount of focused activity, lack of sleep, kinda like someone on cocaine only barely manageable. Sometimes folks who do rapid cycle, after a few years will start to experience larger psychotic breaks.
You should get professional help. Even just living with someone with a mental illness takes its toll, maybe you could use some help as well. Just a thought from a non professional

Ultimatum, meds/psychotherapy or the highway. It is no help to this person to even slightly enable that process. Also be aware that this is a progressive illness, that means after every episode it will get worse the next time. Tough road. I have two friends that are bi polar and several family members and friends that work in the field aiding dual diagnosis folks.

Feel it all. Talk alot. Let yourself be lead to the next place in your life. Kill bitterness (bitterness just means that in someway you are denying what happened. Which never really helps). Avoid romantic comedies, get a neurotic pet (they are excellent at distracting). Take up cooking things with alcohol in them, but just drink the alcohol instead. Smoke some dubage and watch The Mighty Boosh. Write about it. Dance whenever you are angry (it just makes it that much less serious, especially in public)

It is not whether or not to love that is the question. Rather: what does your head tell you to do with it?

Sorry, just read this one and have to change my response a bit.
Oh how we all get accustomed to the way we communicate with our partners. When that changes, so many of us want to think the worst. Usually all that is needed is that change in the way you communicate with him. Life happens, things change we question ourselves. We forget what it was that really was exciting about that person upon first meeting or connecting. For me in the past having gone through times of questioning, it has been difficult to maintain the right kind of communication in relationships. What has most effectively gotten me out of those "funks" was something fun, free-spirited, honest, bold, or simple silliness.

What I am saying is try to break the momentum of fear and all the bad possibilities by doing something different.

Make it fresh. Do something new. Something you know would pull him in, then go from there. If it is all still the same.... I am a firm believer in direct and clear communication. List it out, be honest and hope for the best.

Fish Tail IPA
or
Total Domination