Wasn't this just translated as "happily" in another sentence? Why would this now not accept "happily, he forgets the pain?" Is it actually wrong?
Not wrong, although weird here.
Does that English sentence make sense though? Could you give me a context where you'd say that?
His girlfriend left him, but he found a nicer one.
If you can forget about something that's hurting you, be it physical or mentally - then that thing wont affect you as much.
is "de" necessary here?
it isn't, you can say "Felizmente, ele esquece a dor"
Thank you very much!