How can I increase my appetite and gain weight after my pregnancy?

Hello doctor,

I am looking for guidance on what else I can do about my weight. I am a 25-year-old female (5 feet 5 inches in height and weight 105 lbs). In high school, I weighed around 120 kg, which has been my desired weight for the past year. After my first pregnancy, I was 115 lbs, and I continued to lose weight. I started losing hair and became extremely tired all the time. I went to my family doctor, who told me it was normal since I was breastfeeding. A few months after I stopped nursing, I went back, and they did a full blood workup and only found that I was vitamin D deficient. It has been ten months since I stopped nursing, and I still cannot seem to gain weight. I have no appetite in the evenings and normally only feel hungry during lunch. When I force myself to eat, even when I am not hungry, I feel sick. I am still extremely tired and feel weak all the time. I drink meal shakes to try and help, but the heaviest I have been in a while is 107 lbs. I feel like this is a never-ending battle. I can easily count my ribs while looking in the mirror, and my collarbones stick out far enough. Is there anything I can do to help with my appetite?

Thank you.