What should I do about feeling extremely guilty for leaving my last job and never returning for a farewell party.
Why didn't you return?
I needed to travel, I wanted to go with my family. But right now I'm back to my job. Although the party is over now.
Although that's nice they threw you a farewell party, you shouldn't have to go if you don't want to. You were busy. It's okay and you don't have to go back to an old job.
I don't know why the company didn't bother, it's pretty rude and unprofessional not to have one. But I doubt the new company will do one.
It was considerate of  your job to throw you a farewell party, it shows that they care. It would be kind of you to sincerely apologize if you feel bad. This may help you to move on and move past what happened.