I think the "what are you trying to discord" question is the most important one here. After you pass the early levels of any skill (about 30 I think), you can only gain if:
1) You succeed in your attempt
2) You have a chance of failure in what you're doing
So, if you're trying to discord something and never succeed, you'll never gain. And if you discord something that's too easy where you have no chance of failure you'll never gain. Remember for disco, there's a music check & a discord check, so if your music is super low as well, you'll have a tough time succeeding at all. Of course, you can get GGS gains if you're not using the skill properly, which looks like what you've been getting.
So the bottom line is, find something to train on that is appropriate for your skill level. Having exactly 100 music is great advice, and using a non-exceptional instrument is nice too. But at your level in the mid 30s, picking the right target should be all you need to gain swiftly.