@melinda561 Hey Melinda, sorry it's taken so long to get back to you. I had to give it some thought. I've done a lot and wanted to distill what I think worked for me.
The first thing for me was to track my weight and meals and calories. I could see in real time what was happening, when I was stuck and especially when I started to gain. I no longer track meals or calories but I still track my weight every morning. Then I needed to add more calories and lots more protein, first 60 grams/day, then 90grams. I would and still drink OWYN which is sugar free and works for me. Then I worked with gastroenterology and functional medicine to treat and manage digestive issues. That was also key, as I had malabsorption as well as mild malnutrition. I just didn't want to eat much or often. Another key was having prepared food in the fridge, ready to eat, because I was often too tired to cook, and often too tired to climb stairs to get to the kitchen. With all that, I still struggled to gain. What really pulled it all together for me was watching the NJH videos, particularly that verified just how difficult it is to gain and offered a goal of gaining 1lb per month. I thought "that is surely doable." Even with a restricted diet, I tracked and drank protein and ate every time I felt hungry (even if it was midnight). I also started working out with a personal trainer for 30mins/week. After 2 years of being at 16.3 BMI, I started last October with the goal of gaining 1lb per month. Initially, it took a month and I could and did lose it in a day. But I gained it back and kept going. By January I was reliably holding the weight gain. Now, my BMI is 19. My pulm and ID docs gave me fistbumps, asked what happened and when I told them about the 1lb/mon goal, pulm said it's such a low bar! Anyway, it has worked and I work out 3-4x a week now. I feel like I've been dropped into my 35yo body. I also feel very fortunate. I can and do cook now, starting with a big breakfast every day, and simple easy dinners or crockpot soups/stews that last for several meals.

I hope this helps. Wishing you the very best with your journey,

Roisin