Many women have stopped treatment early due to the nasty side effects. And some, like me, tried only a few doses and said "Uh unh - I ain't doin' this." If you look back through the messages on the breast cancer forum, you will find dozens of concerns similar to yours.
Be patient as concerns the weight loss - it will happen, but it will take time. The weight gain was quite possibly due to severe estrogen suppression. Taking Arimidex chemically produces just about the same effect as if you had had a hysterectomy and your ovaries removed, or had gone thru menopause. More women than not report that they gain quite a bit of weight after hysterectomy - 50 pounds is about the average. (I speak from personal experience!!) And just so many women report weight gain when on the anti-estrogen drugs. This seems to be the body's attempt to increase estrogen in the body. Fat cells produce estrogen - the more fat, the more estrogen - so the body starts gaining weight by increasing fat storage.
It will take perhaps 6 - 9 months for you to metabolize and totally eliminate the arimidex from your body. As this process occurs and your body goes back to the estrogen production that would be normal for you at your age and reproductive status, your system will less and less perceive the need to maintain the extra fat cells for estrogen production, and you should be able to start losing the weight.
If you make sure to incorporate lots of cancer-fighting foods - like cruciferous veggies and fresh raw fruit - in your diet, that will also help balance out the effects of the arimidex. Many foods like broccoli, asparagus and brussel sprouts are also very gentle natural estrogen regulators. Also increase your exercise as tolerated, as the side effects of the arimidex begin to lessen. make sure to focus on gentle, low impact exercise, such as walking or swimming - this will help to strengthen your muscles and bones without overstressing your joints. It also increases metabolism.
Make sure to drink lots and lots of fresh pure water. The body absolutely has to have it to promote weight loss, and for healing from the detrimental effects of drugs.
You might consider getting a rebounder (a 36" mini trampoline.) It's wonderful exercise for weight loss, and you can start with very gentle bouncing and work your way up to a more vigorous routine as your body heals. This is an especially way for women who have had lumpectomies or mastectomies to exercise, as it promotes lymph drainage in the arms and breasts.
http://www.alkalizeforhealth.net/rebounder.htm
Sincerely,
Tre