What should I do when I don't agree with the feedback from my boss?

This varies depending on the person, but here are some general ideas!

• Try to get a different perspective, if possible from someone who can be neutral, and is familiar with both you and your work and your boss. Depending on your relationship with your boss, sometimes it can be helpful to hear someone else give similar feedback to you so it doesn't seem like it is just coming from one place.
• If you're not getting this already, ask your boss to give you specific examples or things they would like to see as a result of their feedback. And, try to identify what type of support you need in order to do those things. For example, maybe scheduling a regular async check-in so that you can get more real-time feedback so you know if you're moving in the right direction or not. 

That's a tough one. A few ideas:

• Thank your boss for the feedback and ask him/her for suggestions on how to improve. Ask for specific examples. Ask "what are others doing that I am not?". If I were your boss in this example, I would want to know that you are struggling with the feedback so I can help provide clarity.
• Find a mentor within the company (or potentially outside the company) that you can go to for advice. Share that you are struggling with the feedback and ask for ideas. A good mentor will be able to help.
• If you have a peer that reports to the same leader, and you have a good realtionship, you can speak with him/her to see if they are getting similar feedback. They may be able to help interpret the feedback.