[M19] Having a hard time gaining weight

I'm 186cm last i checked and currently 64kg, so my BMI is 18.5 which isn't optimal. I wanna try and reach 75kg as a long term goal, but I can't seem to really gain that weight.

I don't feel good about it at all, and I'm really struggling. I try and eat as much as possible. I eat big portions, sometimes twice as much as my other family members (who are all adults) It's frustrating.

What foods have helped you gain weight? Preferably cheap foods. Being healthy is optional, I just wanna gain weight.