Any dieticians?? Doctors?? Insight on GAINING weight? HELP🆘

Or anyone who knows about gaining weight-

So i’ve been dealing with my weight yo-yoing for about 3 years now. I have the opposite problem as most people do and its been extremely hard finding resources so please give me as much in sight as possible...

Ive been roughly 110lbs most of my life and i’ve wanted to gain weight... The most I weighed was about 145lbs and I would love to be that weight again. I currently weigh 120lbs and I cant seem to get the weight back on no matter how hard I try.

I’ve been dealing with really bad bouts of cyclic vomiting and I cant stop losing weight. I had an appointment to get my thyroid checked and I’ve been taking other medication to ease my anxiety and help with my vomiting. But now I want to start GAINING and maintaining the weight.

I have a few apps to help remind me when to eat but I dont know what to eat. *I cannot eat eggs* I have a severe allergy to anything made with eggs. Is there any supplement that will help give me an appetite? And also any kind of supplement/ powder that will help me up my protein.

Im anemic and plan on starting an iron supplement regimen with vitamin d.

Thank you in advanced!!!

Im in my early 20’s

Im 5’3

Im 120lbs trying to gain 20-25lbs

I moderately work out