Why am I not losing weight?

This is all super helpful, and I do feel like I have replaced some fat with muscle--I'm definitely a lot stronger than when I started out. I noticed a few years ago, I dropped twenty pounds in 2 months from going low carb and doing an hour cardio every day. I feel like my body is very resilient to weight loss--is that possible?

No, it's not.

When you have a lot of weight to lose, you drop the first 10-20 pounds fast, then all but stop. That's normal for everyone.

And unfortunately I have a very strong suspicion that a lot of that 20 lbs is lean body mass, from you eating so little. I ate 1200/day for a good year, and lost so much muscle it was sad. And I'm only 5'0". You need to eat much more than you're allowing yourself. Yes you're losing weight, but not at a healthy or sustainable rate, and most likely not the right KIND of weight. Find out your TDEE and eat 20% less than that. You'll be shocked at how much you'll be able to eat and still lose weight.

Oh, and unless you LOVE cardio, there's no reason to slave away doing an hour every day. 10-20 min before lifting weights as a warmup is more than enough.

I agree. You have To eat at a specific type of surplus to really gain muscle mass plus, (no offense) but a Jillian Michaels DVD isn't going to give you the amount of strength gains to show up on the scale. You are probably getting stronger, which is great, but not gaining mass. I don't want any one to get to comfortable worth the "I must be gaining muscle weight" explanation. I lift heavy, have seen my max on squat go up by 50 lbs to over 200, and I haven't gained weight, but have seen a slight body comp change. My legs are getting bigger though, which isn't my favorite, but it is in my fitness plans for now. Anyways...

My advice, honestly, if you really want answers.. Is schedule a session with a Registered Dietician. An RD. There's soooo much individualized opinion that goes into your diet, exercise and goals that no one on here is qualified to assess. Even if they arer an RD, without seeing you. They aren't too pricey, not covered by insurance unless its a medical visit, but make sure it's an RD, not a nutritionist. In my area it's about 50-75$ for a thorough 1 hour session/analysis with follow up. That's about the price of a therapy session, and to me it's worth the good info a d the piece of mind over what I can google/post on forums about. I suggest it bc that's where I'm at in my journey too. You can google the hell out of the topic and get 100 different answers. I have a ton of respect for the profession, and feel like they really do see the whole picture better than any forum ever will. Good luck, eat right.