There are many possible causes for mood swings. You are going to need to be a detective and rule out whether the problem is emotional, medical or a combination of both. From an emotional standpoint, your girlfriend may have a borderline personality, which means that she sees the world and those in it as either all good or all bad. When she is angry at you, for example, you become a total, 100% monster; and when she is feeling loving toward you, you are all perfect. For the borderline personality, there is no middle of the road, only black or white, which can explain the great highs and lows in her mood. This type of personality disorder requires long term modern psychoanalytic treatment, which she isn't likely to receive since she is against therapy. The fact that she is so adverse to therapy makes me think that she has a problem that she doesn't want to face.
She could also be suffering from a chemical imbalance. You may have heard of manic-depressive illness in which a person vascillates between extreme highs and extreme lows. Highs can last for weeks, months or years, as can the lows. Cyclothymic Disorder is a related condition in which moods fluctuate more frequently. She may be suffering from this condition, which is often treated with a drug called lithium.
Mood fluctuations are also commonly caused by blood sugar imbalances. Approximately 25% of the population secrete excess insulin when they eat carbohydrates (bread, rice, pasta, sugar, etc. ). Carbohydrates elevate blood sugar and the purpose of insulin is to bring the blood sugar down. If a person secretes too much insulin, the net result is that the blood sugar actually goes too low and the person experiences a blood sugar crash. When the blood sugar drops, the mood crashes with it. During a crash a person will feel irritable, angry, moody, sad, etc. . Blood sugar imbalances are such a common cause of mood fluctuations and yet most doctors, psychiatrists, and psychologists don't think to check for it. High insulin not only causes low blood sugar attacks. Left untreated it can lead to clogged arteries, heart disease, high blood pressure, high cholesterol, obesity (fat on the gut is a common side effect of high insulin), and inflammation throughout the body, including joint pains, and much more. High insulin levels are not easy to detect.
Thankfully a company called Ideal Health can determine whether she has this problem or not using a simple, in-home test. If they find that she has the problem, they will show her how to change her eating habits so as to prevent the sugar highs and lows. They will also give her a supplement to help stabilize her blood sugar (and by consequence, her moods). You can find out more about how to order the test by going to the Wellness Dictionary section of the site. The name of a contact person from whom you can order the test is listed there.
The bottom line is that you need to treat this problem as a relational one since she is having an effect on you. You need to tell her that she may not believe in counseling, but she needs to believe that her behavior is having a bad effect on you and the relationship. She needs to know that if you aren't happy, then she isn't going to be happy either--especially if and when you are forced out the door! You need to have her agree to work with you on solving the problem. If she isn't willing to take responsiblity for her behavior and the effect that it has on others, namely you, then you are going to have to study what you get out of being with someone who mistreats you.
When you understand more about what purpose this relationship serves for you: to relive a familiar pattern of childhood abuse? to keep you feeling like a wronged victim, so that you have an excuse to be angry? to give you an excuse to keep your distance (are you afraid to get too close to another?). When you understand more about what you are obtaining by staying in this situation, you will be in a stronger position to decide how you want to handle the problem: either by doing nothing and staying a victim; by getting help for yourself and healing your childhood wounds. When you work on yourself, you should feel more ready to put your foot down and demand that she work with you on improving matters. Let me know what happens.