how do i just say no......

hi guys

so im in a bit of a situation and its causing me abit of grief. some of it is my fault but i dont really know how to fix it.

i am a full time carer as well as being a primary carer for my two younger sisters and imstruggling with the pressure to keep it up,

ive been diagnosed with ptsd, depression, gad and a few others but arent overly important at this second ( health anxiety and social phobia). just thought id meantion these just in case they are relevant 🙂

but anyway back to the point.... i dont know how to say no.... and if i say no i feel really bad about it despite struggling with my own issues and pressures

how does everyone just simply say no? esp without feeling bad about it?

even the small things such as my sisters saying 'can you take me outside so i can ride my bike? i cant say no to and feel extremely obliged to just do it. even if im trying to do some study or something else i just seme to drop everything to do what they want. i feel really bad if i say no and i think they will hate me.