First off, I KNOW improv! Pretty much anything I play is improvised. I never bother learning a song note for note. Except the main riff and chord fingerings. I just learn the key, scale and make something up that sounds like the song, or whatever. I too had that problem at first, but the advice I have is just, get comfortable, relax, never hesitate, and jam! emphasis on the hesitate part! Dont just half-@ss a note. if your going to play a note, than stick to that note and play it! Hope this helps!