Learning to say no

For my entire life I have struggled with this concept that no is a response enough. I am trying this now in my life and I'm finding myself so anxious. The last two times I have said no it has left me unable to sleep at night and pretty worked up.

Example

Scenario one: Neighbor texts me "What weekend are you free in September to come over and do a foraging class with others?"

Response: sorry I cannot commit to anything right now with my schedule.

They processed to text me all about how they remember when their life was busy with kids. The text was like 4 pages long. I was up at night just stressing about how my next communication will be with this person as she never takes no for an answer and makes me super uncomfortable.

Scenario Two: Aunt messages me after I've been home sick with two toddlers and we all have covid. "Hey your mom's stuff is in my spare bedroom when can you pick it up and your grandma's as well."

I responded to ask them about their things I can't help right now. They sent back a disgruntled text about how they want their room back. My mom is paralyzed and can't move it downsized so has no room. Not sure how I became the go to person for her as well but I have no time or energy or space. This is causing me so much stress just being asked as I don't say no normally.

I become the keeper the go to the helper the arranger and its exhausting. How do I say no to these things without guilt? I have two small kids no childcare and my husband and me both work outside the home. We never see eachother and we are run ragged. All our family is elderly and is in need all the time. There is no leisurely time. No vacations. No dinner even together as our work shifts are separate. We literally have no time for all the things we need to do in a day and we are so beaten down. When I try to tell people this they just don't get it. Then I have guilt for saying no. I'm absolutely exhausted and can't sleep because my aunts spare room has belongings that don't even belong to me in it is ridiculous.

How do I say no without guilt?