Consider this. To lose 2 pounds a week you need to create a calorie deficit of 1000 calories a day, and 1500 calories a day to lose 3 pounds a week. You currently consume 1000-1200 calories a day. Without considering exercise, to lose 2 pounds a week at that intake you'd need to cut your calorie intake to virtually nothing, and for 3 pounds a week you'd have to eat nothing and exercise even more than you already do. Considering you already exercise a lot, how would you go about accomplishing your goal, and if you did, you would risk losing muscle, losing body fat, and perhaps risk malnutrition. Excessive loss of body fat, or very low body fat, is associated with loss of menstruation, low bone density, and other medical conditions. As a nutritionist, would you counsel your patients to do this? To answer your question if losing 20-25 pounds in 3 months is unrealistic, yes it is, and it could even be unsafe. As for your eating, and this is always very sensitive, but you say you do not want to be anorexic nor do you intend to be. I would guess that no one would. However, if you read your post carefully, it is of concern. You say you are scared you undereat, you count the number of chips, a treat is an apple with peanut butter, and so on. This sounds uncomfortable emotionally, and must be exhausting. You don't need to be anorexic to check in on one of the eating exchanges, and so I recommend that you at least check in over there and see if you can get some insight and advice on how to proceed.
http://boards.webmd.com/topic.asp?topic_id=1135 As for the weight gain over night you are retaining fluid, not gaining weight from fat. If you ate a jar of pickles before bed you'd wake up 4-6 pounds heavier just from fluid due to the sodium content and fluids. My suggestion for you is to do your best to shift your focus away from the scale and focus instead on maintaining muscle and strength, and shaping your physique. This may be a tall order, but you deserve to feel good about yourself without starving, and perhaps building a strong and toned physique, at any weight, will help.