so sad. no weight loss

Opening your diary will help. It's possibly water retention but it's hard to say. Has your doctor done blood work for you?

Personally I think you're overestimating your calories. I see you use a HRM but there's no need to eat back normal walking unless you're morbidily obese. I'm normally one to suggest eating more calories but it doesn't appear to be working for you. Why not try two weeks eating at your 1500 calories and see if anything changes?

1500 cals sounds like quite a lot. Have you tried cutting it down to around about 1300?

I feel your pain. I've lost and gained the same six pounds in that time. All I can do is keep plugging away and know it will break eventually. My diet has really slipped the past couple of weeks as well so I feel your pain.

Several things come to mind. I don't want to bore you with this long post, but here are some ideas...

1. Nutrient ratios
Yes, if you open your diary, you might be able to get more specific advice about how to tackle this. I know that for some people, ratios of carbs-proteins-fats are what makes or breaks their ability to lose weight. That is what I had to do, but I was a total skeptic that ratios matter until a couple of years ago when I tried it out for myself. Here's an anecdote. Bear with me... My trainer is a 60 year old woman who is incredibly fit, and keeps her body fat level at around 20%. She does 12 week boot-camp classes at my fitness center. The fitness center gives each participant a pamphlet from the USDA on healthy eating as part of the 'getting started' package of the class. Several times now, she has personally used those USDA recommended percentages (higher carb, moderate protein, low fat) while she taught the class, so that she would be able to really connect with her classes and experience what they experienced on the program. She tests her body fat level before and after each class cycle. This is someone who doesn't eat junk carbs or junk fat - whole foods, healthy eating, no processed crap, and lots of organic foods. She found that every time she switched to the USDA -style eating plan, even when she kept her calories the same, she gained fat. Every time. Up to 3 percentage points. When she eats closer to a 40-30-30 style, she loses the fat and stays lean. Whenever someone in the boot camp class starts to stall with weight loss, she quietly pulls them aside and suggests they start incorporating more protein and more healthy fat, with a little less carbs, to see if that helps... and she tells me that more than half of the people that do this end up shedding fat again. It doesn't work for everyone, but it definitely works for me.

2. 50 lbs... says who?
Now, your doc wants you to lose 50+ lbs... is that based on BMI? Have you tested your body fat levels? If your doc is just going by the numbers and not taking into account your frame size and muscle mass (which BMI completely ignores), that might not be a realistic target for you. My doctor wants me to lose 60 lbs... my trainer wants me to lose 4% of my body fat, which would be roughly 30 lbs. My doctor is just looking up a number in a book and not using any of her professional judgement... my trainer knows my body composition and structure and is personalizing her recommendation. If you're going to have your body fat tested though, don't waste your money on scales or handheld analyzers... call around to different gyms to find someone who is good with calipers.

3. Measurements?
Are you taking your measurements? This is so important. You will not get the whole picture about how your body is changing if you don't take your measurements regularly. My trainer does mine (because I want to pull the tape tighter all the time and cheat) but you could have a close friend do yours.

4. Adaptation
For three months you've done cardio and weights, and yet your body isn't changing... it could be that your body has adapted to that style of work, and has adapted so much that it's not going to make any more changes if you keep using this current routine. So, change it up - if you are doing steady cardio, start using an interval program. Change your weight lifting to high rep-low weight or low rep-high weight. Or cut back weights to once a week, and do pilates/power yoga for the other two days. Get some kickboxing dvds from the library (or buy used ones from Amazon). Ride a bike instead of running... etc, etc, etc. It's recommended you make a change to your workout style at least every 8 weeks... so try something new for a couple of months, and then go back to your old routine to keep your body guessing.

5. Portion fine-tuning
Your body has already made some good changes from what you said, and it might need even more discipline to keep losing weight. Study after study as shown that most people are just not good at eyeballing their proportions. Using measuring cups and spoons is helpful, but what will keep you REALLY honest is using a small digital food scale. Don't bother with a dial scale, they are not nearly exact enough. You need a scale that will measure each gram of salad dressing and cheese, not something with a dial that will give you only a rough estimate of ounces. Taylor makes a decent one that costs about $30, Amazon sells it for $19, and local Target/Walmart/Ross Dress-for Less might even have it for less than that. It's an incredibly useful tool.

6. Maintenance Matters!
Being in a plateau is beyond frustrating. But, maintenance is just as important! If logging your calories and workouts has helped you maintain your weight loss so far, don't jump ship now - it's just as important for you to maintain your progress. You know this works - you said yourself, you're halfway there! Logging is a fairly simple task, it's only a few minutes of your day... if this is what you need to stay accountable and keep that great progress you've made so far, then please, keep it up. Losing is only half the battle... the real challenge is maintaining the loss. So keep checking in and being accountable to yourself. You are worth it!

Good luck!