As others have said, talk to the professionals as there are many reasons for unintended weight loss, feelings of fullness etc.
I have a similar stature to you – I’m always borderline underweight or underweight. I recently noticed I had lost 5-6kg despite eating the exact same and no change in (in-)activity. I brought it up with the GP who said it was probably muscle atrophy. I’m unconvinced.
I did lose a significant amount of weight a couple of years ago (about 12kg) but that made sense as I was able to eat very little for months. I needed a dietitian’s help to put that weight back on – it did not go back on when I returned to eating normal amounts of food. The dietitian told me to increase protein (that was the most important one – it had to be ¼-1/3 of each meal), increase fat and let those two squeeze out fruit and veg for a while (because they’re low-calorie but make you feel full, so they leave less room for high-calorie foods). Ideally, I would have been having calorific smoothies during the day but they didn’t work out well for me. I did snacks instead but more liquid calories would have been good. I was amazed that I had to keep adding more and more to put the weight back on. But I did, and then I maintained it for ages and was able to come down to more normal amounts of food and still maintained it, so I stopped weighing myself.
The go-to for dietitians is to get you to add dairy - milk, cream, cheese, yoghurt - into places they don't normally belong, but you mention you don't do well with them. Me neither. Most oral supplements are based on whey protein but the dietitian should know if there are others. There weren't where I live but there might be where you live. I found peanut butter, walnut butter and almond butter helpful. (Peanuts and walnuts are low-FODMAP so if you have FODMAP issues they're still likely to be good.) Oh and I pour olive oil on things too.
This time around I’ve managed to put on 2kg by myself, without making myself feel uncomfortably full, but have stalled there. So far I haven't stuffed myself as much as I did the last time as it exacerbates other issues, but I am weighing myself every week to make sure I know what’s happening, and I have another appointment soon that might help.
It’s well worth talking to a dietitian to get advice as having the right balance of macronutrients is important.