There are two phrases that have revolutionized my life in this area. The first is "Sometimes you have to say no so that later you can say yes." We know this with kids, that we have to say "No, we can't go to the water park, we have to go to school so that this summer you're not in summer school and we can go to the water park then." As adults we have to set those same boundaries for ourselves. Our culture almost glorifies being stressed out, stretched thin and running on fumes. We have to know our priorities, set our boundaries and then defend them like our life depends on it, because your health really does.
The phrase I've found that helps both myself and others respect when I have to say no is, "That sounds like a fantastic opportunity but I just don't have the margin." Margin is the breathing room, margin is the wiggle room, margin is the empty space. Sometimes I do feel guilty, and if it's something that I REALLY want to do, I might try to rearrange things but sometimes you just have to say no. You not having margin doesn't mean that person isn't important to you, it just means your plate is full. It's a complete explanation in itself.
I'm not sure if you're familiar with the analogy of the spoons, but it's a great picture for people with auto-immune disorders or other invisible illnesses to gauge how they're feeling. You start the day with a certain number of spoons, and everything you do takes a spoon. But not the same amount of spoons. For example, having this conversation with you online is like 1/2 a spoon. But if we were having it face to face that would be 2 spoons. 4 if it were on the phone. For me, taking a shower is like, 8 spoons some days, and I have to go lay down afterwords. Everyone's scale is different, and some days you have more spoons than other days. When I'm reaching my limit or something is just too much for me to process, with people I am very close to I tell them, "I am sorry but I just don't have enough spoons for that." At first it was awkward explaining what I meant but now they know and understand. I've yelled "NOT ENOUGH SPOONS" at the top of my lungs while driving down the interstate before lol.
Do either of those phrases seem like something you could incorporate to help say no? I'm proud of you for opening up this conversation, because boundaries aren't popular. I'm also excited to see what other people say!