You should have been able to master saying "no" previously so now it is time to tackle the uncomfortable task to ask for help. We like to avoid looking/feeling weak. Asking for help shows strength; you know your limits and that we can't do it all ourselves. People who care for us want to help.
Change your mindset: Let go of that thought you have to do it all. We are all worthy of the gift of help.
Reassess your priorities: Keep in mind what is most essential to you and consider how much time you're giving to that thing or person. Do you need to rearrange some things to that thing or people that matter most to you?
Be specific: Make sure you are clear with your helper about what you need help with. It may also be a good idea to be clear about what exactly you need to avoid further frustration.
Request training that speaks directly to self-care: Ask school counseling staff to provide training that speaks directly to this. Maybe even have a schoolwide wellness program will help make a difference.