I learned a a song about a month ago and started practising it.There was a time i used to play it quite well but three weeks ago i just started playing it worse and worse day after day.Now there is a part on this song i cant even play!!!Why does this happen???
You've most likely hit a rut in your guitar playing. Change the way you're approaching things, and you'll get out of it. Learn a new song in a different style or just work on different things you're not used to.