Coming back to the boundary board, it's a good idea to start with boundaries that are with yourself. Starting out with boundaries with yourself will help you practice saying "no" without the possible confrontation of another person. You need to practice holding yourself accountable before expecting to be able to hold another person accountable.
To start, think of something small that you can begin with. Make the boundary with yourself and then think of a consequence for crossing it and a plan to follow through. For example, maybe you need to get more sleep, but you are in the mindset that you absolutely have to do a multitude of things before you can go to bed. Lack of sleep is affecting your mental and physical health and you know that you need to make this change. A plan to approach this boundary you've set for yourself could be changing your routine to be more productive at a different time. If this isn't a workable idea, you may need to learn to prioritize the tasks you need to do. Give yourself some wind-down time and try to relax. Even if you're not sleeping yet, rest is good for your body and mind. If you find yourself continually crossing this boundary you've set for yourself, you may need to cut out a task or activity to make sure you give yourself the time to get, or at least attempt to get, proper rest.
Boundaries are hard, even for yourself. However, it's very important that you make yourself a top priority for your long term health and setting boundaries for yourself before implementing them with others is a good idea. Often enough, people that don't have healthy boundaries and are trying to learn them have a history of trauma in their lives. Due to trauma and mistreatment, we become our own biggest critic. Once we can say and enforce 'no' to ourselves, we can feel more empowered and begin our journey with setting boundaries for others.
Hang tight, you can do this!