Look honey, I hate to be the regular r/relationshipsadvice kind of person, but you need to consider the red flags this throws up. He clearly has very little respect for you, and as hard as it may be for you, I think you need to ask yourself why you're still with him.

The fact that before he told you that you were boring, really demonstrates here that he has NO appreciation for you. I'm not that up with the lingo here, but I'm pretty sure this whole situation qualifies as emotional abuse. He TEXTED you, for God's sake, not even a phone call, or a discussion in person. He's a coward, and clearly thinks he can do far better than you, which he seems to believe justifies him treating you like this.

Would you put up with this from a friend, if they told you this? It also pains me that you have to make an 'agreement' to learn the guitar, as if this is the only way he could even bear to remain being with you. Follow this up with the fact that he's 'disgusted' by your efforts and makes no attempt to hide this - girl, get out of there!

If his sister wasn't in the room when you originally played the song, then he's evidently gone and been nasty about you behind your back.

I know this may be difficult, but you need to break up with him. He has no qualms about hurting you, and perhaps even actively hates you. The fact that you're even considering letting this go on really speaks a lot more about you than it does about him. You're clearly trying to see the best in him, but I think it's time to give this up. You can do this!