Trying to gain weight

Ive struggled with my weight ever since my 1st kid 5 years ago. I was always thicker and after I breastfed and went through a break up I lost like 25lbs.. (I'm already small) I've put some of the weight back on over the years but my body just won't really go back. I'm now pregnant again and want to gain as much weight as possible. The catch is I'm going through a breakup again with this baby's dad and can't eat. Any advice? I just want to gain a healthy 20lbs or so.