1. Don't observe others who are doing what you dream of doing
Imagine you are an aspiring baseball player. Do you think you would ever watch a
major league game? Watch YouTube videos of great plays? Follow your favorite team? Nah! What is there to learn from someone who's already achieved your dream? Don't waste your time attending concerts or shows, listening to the type of music you want to perform, or observing a mentor. Save your energy, keep your head down, and stay in your own little bubble. You can learn so much more from your own experiences.
2. Surround yourself with people who downplay your dreams
If you want to sabotage your dreams, surround yourself with people who mirror the critical voice in your own head:
"You're not good enough"
"Don't quit your day job"
"Maybe you should take some accounting classes, just in case"
"You missed a note...or two...or twenty"
"You always forget the lyrics"
"Your dream is out of reach"
3. Ignore your teachers
Just because your teachers have been performing much longer than you, they were hired by your esteemed school/studio as a professional, and/or they have taught many students over the years -- that doesn't mean they know better than you. Just go with your gut and ignore how your teachers are trying to mold you into the musician you want to be. You know better than them.
4. Don't spend any money to support other artists
It's useless to pay to support other artists. Don't buy their recordings, don't attend their launch parties, don't buy tickets to their performances, and don't help them prepare any of the above. When it comes around to your turn to sell your music to others, they'll be bound to show you zero support.
Refuse to teach, refuse to perform any genre of music that is not your favorite, refuse to take gigs that don't pay your ideal wage, refuse to perform with individuals or ensembles who are beneath you. Stick to your guns, and hope and pray that your very narrowly-defined dream will come true. And whatever you do, don't expose yourself to any new genres, techniques, or ensembles that might help you grow and become more marketable.
Your reputation with your teachers, directors, and colleagues will blossom if you are continually late and/or unprepared for lessons, rehearsals, and performances.
8. Skip opportunities to perform or audition if you're scared
If an opportunity falls into your hands but makes you scared, then just let it slip through your fingers. Another similar opportunity will come along very soon when you are completely prepared and free of anxiety. The individuals offering opportunities will continue to ask you indefinitely to see if you're ready yet, so don't worry about telling them "no" a number of times. They'll keep calling.
9. Sacrifice your health
Good sleep, whole food, and regular exercise are insignificant bothers for musicians. None of those three things will impact your performance or attitude. Singers are especially immune to the effects of sleep, diet, and exercise, since their "instrument" is completely unrelated to their body's health.
10. Work half-assedly
If you are serious about sabotaging your dreams, don't put much effort into the tasks it would take to get you where you want to go. Do just enough to keep you in the game but not enough to get you to the next level.
11. Set goals, but don't write them down
You have a 10 times greater chance of achieving your goals if you write them down. so in order to derail your dreams, keep them hidden inside your head. Whatever you do, don't journal or God forbid (!) make a vision board to create a visual representation of your dreams.