If you have played for 3 weeks and you can't get Jingle Bells, its time to take a break. Like Anders said, do something else. Maybe back to the very basics. I have played guitar for 13 years, and I am JUST NOW (literally like a month ago) going back to learn the notes. I have used tab for all these years so its brand new stuff for me. I can play some of the fastest solos out there, but I cant play simple songs (like you with Jingle Bells) at a slow pace just because I'm having to learn the notes now.

It's frustrating to know that it seems like such a simple thing, to see a note and play it, but doing something new is always hard. Just keep practicing.