Metronome's just keep steady time. With the beats, you play notes -- whole, half, quarter, sixteenth, etc. From there you can play triplets, etc. Metronomes are great at keeping time and really help musicians as they practice "in time." This translates over in your total skill and ability to play with others. You have to have a good feel and understanding in rhythms too to really utilize a metronome. Either way, too many MUSICIANS, not just guitarists, fail to use metronomes. This is how you get sloppy -- by not using one. Timing issues can arise, as well as overall playing skill - utilizing rests, different rhythms, etc.
I know others have offered you empty tidbits on here -- like "just use the Phrygian mode ..." That will do you no real good if you don't understand theory in general -- as in, why you are playing what you are playing. Forget about Paul McCartney and Keith Richards. That isn't the norm. Those guys won the lottery with their talent and opportunities and timing. Music, like anything else we try to do well in life, requires some education.
You can get a teacher or opt not to have one. Either way, a good theory book is necessary. Don't just use Phrygian as was discussed earlier. It is fine advice but it will only get you so far IMO. If you want to understand why things work and have an easier go at writing music, study theory in its entirety. The books I've listed are great, but there are be plenty of others to choose from too. It isn't rocket science. It just requires some patience, application, and effort. That's it!
If you ARE just going to use the Phrygian tip, remember the relationship between the notes. CM (M=major, m=minor) scale is easiest because there are no accidentals involved -- meaning there are no sharps or flats to deal with. Everyone knows the major scale -- do re mi fa so la ti do. We've hummed that since our youth. If you transcribe that to guitar, you'd have the following intervals after the starting (root/tonic) note: whole, whole, half, whole, whole, whole, half (WWHWWWH). If you did this correctly, you should end up back at your tonic, just an octave up. This scale, known as the Major Scale, is also known as the Ionian Mode (Note that not all modes are scales, and not all scales are modes).
All modes are then is playing within the interval pattern of the Major Scale. If you are playing in the C-Major Scale, and play D within the modes, you will be playing Dm Dorian. Anyways, back to what I'm writing ... The first mode, the Ionian Mode/Major Scale is listed as the I (upper case Roman numeral means it is a Major interval. Lower means a minor interval). The second mode, called Dorian Mode, is a minor interval and is listed as ii. This designation of "minor interval" means that the sound of the mode is very close to what we call the Natural Minor Scale, which is the 6th mode (Aeolian). What makes any scale or mode minor is based solely on the third note of the scale. Using the Major Scale, anytime we flatten the third, we create a minor interval. The Dorian Mode looks like this: WHWWWHW.
Notice a pattern? Yup, the Dorian Mode just starts on the second note of the Ionian Mode. Here are the rest of the modes:
Phrygian (iii) HWWWHWW
Lydian (IV) WWWHWW
Mixolydian (V) WWHWWHW
Aeolian (vi) WHWWHWW
Locrian (either depending on what you do to a few notes) HWWHWWH
Locrian is a mode, but not really used in popular music. More so, it can be found in classical music, as well as avant garde and art styles. I can't do diddly squat with that mode. Play it and you'll see why.
Do you know your Pentatonic Minor scale? That scale is comprised from your Aeolian Mode, without the 2nd and 6th notes. The Major Pentatonic is your Ionian Mode, minus the 4th and 7th notes. That scale, if you play it to the octave and were counting out eighth notes, is your, "My Girl" progression.
Confused? Good. Get a book. They will better prepare you for understanding the basics (in a few chapters), before diving into such complications.
One more thing. Ever wonder why the ii chord in a Major Scale progression is minor at the very least? If you were in the key of C and playing a I chord (CM), the notes you would have would be the Tonic, 3rd (E), and 5th (G). The next note in our Major Scale in the key of C is D, or the ii chord. We know this note is minor because there is a flatted third in the scale. We also know it is a minor interval by the way it sounds to our ears. All you have to do to know what notes makeup the ii chord, is to move all the notes from you previous chord (CM/I chord) up one note. The C would become a D, the E an F, and the G would become an A. Notice you are only moving up to the next note in that key's Major Scale pattern. When you play these notes as a chord on your guitar you have a Dm chord. Look at your 3rd note, the F. Normally, a D chord would need a third that was a F#/Gb for it to be a major sounding chord. Yet, keeping in line with our scale that we are in, the ii chord/second note of the C-Major Scale is minor. Interesting how this works, isn't it?
I think this should suffice for the moment. Don't even get me started on the 7th chord. Too much to digest perhaps ...
Learn theory! IT IS USEFUL! May the force be with you my friend ...
I've never taken a "break" from music, ever. I've always stuck to somewhat of a loose regimen, even if it wasn't shredding scales. At least getting in some good writing time, working on feel and vibrato, etc. I have always continued to improve and I hope to keep doing so. I'm still hungry to learn and to create, and that's what important IMO!
Key of D major has an F# and C#, with the rest natural, while D minor is D, E, F, G, A, A# (or Bb) C. 
