He told you to only practice 20mins daily? I can mosh out on one solo for 20 minutes with the same scale running the whole fretboard. I don't think 20 mins is ample time to even warm up, let alone get a full practice session in. I took lessons way back in the day and like you I got frustrated and with college and what not I had to stop anyway. I just picked my guitars back up two years ago and started self teaching and I get better and better every day.
Like anything if you don't get better you get frustrated and stop. You can not get better at anything without practice. Guitar IMO takes atleast 2hrs a day. The better you get the more you want to learn the better you get in cycles.
My advice, don't try to learn one song, try to learn what makes a song, chords and scales and timing make a song. If you learn just one song, how are you gona know what you are doing wrong, besides , different folks will play that one song you are learning a total different way than you are playing it. I have found that like anything, guitar requires learning at your own pace and your own style. Don't try to copy don't get in a hurry, just relax and do it. Don't think to much about it.
You should maybe start to self learn, start with blues minor pentatonic scalesl. They are easy and once you have the minors you can see how the majors of that scale relate, then move to modial scales. Do alot of reading, do alot of practicing, do alot of reading. Once you learn one scale ONE note not many; go to youtube and get some backing tracks and start soloing with that backing track.
Learn an A minor pentatonic then look up aminor backing tracks, You'd be shock at what you've taught yourself with some practice and paitence.
Am I a pro, hell no, can many out play me here, oh hells yes. Do I amaze myself, often. Do I have stuff to learn, YES. Does my wife hate my guitars. O YEA lol
Take your time practice, and read and learn and PLAY BABY!