Why is it so easy to lose weight and hard to gain?

I used to weight 104lbs at the beginning of the summer (I’m 5’4 F) and I decided that I really wanted to gain some weight around July. I was aiming to be 125-130lbs. I eat 3 meals a day and a lot of fast food (that I assumed would help me gain weight fast). I have gained some weight (about ten pounds) but I’m still not happy with where I’m at. It is really challenging for me because I work a busy job and sometimes don’t have time to sit to eat a full meal, and I also don’t like to feel “full” either. It’s difficult for me to eat a large meal because I feel sick afterwards. A couple months ago I also noticed that I lost about 2 pounds because work was very chaotic with the pandemic and I guess I wasn’t eating as much as I should have been.

I am just frustrated that I have to put so much effort into gaining and can lose without even trying to.

What can I do to help speed this process along?