A lot of people have a hard time saying "no" to others, myself included. Growing up, I always wanted to please and avoid disappointing others. I have always had a fear of telling people "no." While this may seem great for some, it can become exhausting when you sacrifice yourself for others a bit too much. We have been tricked into believing that saying no makes someone "selfish" or "lazy."
However, saying no isn't always a bad thing and you should probably practice it more. Saying "yes" just out of the convenience of not disappointing someone can have a negative impact on your headspace—increasing your stress level. Realign your priorities and put yourself higher on that list. If you aren't taking care of yourself, you probably won't have success fixing others' problems either.
In addition to respecting your own boundaries, reinforce them by adding the conjunction, "and" after you accept. This will allow you to add any conditionals that may make the task more convenient for you. Asking for what you want isn't always a bad thing, especially if it pertains to a task someone else is asking of you.
Obviously, a balance exists between saying "yes" and "no" too many times. I advise that you try and balance the scales to avoid saying no or yes too often. Don't burn yourself out but don't turn other people away if you can help them quickly or easily. You never know when you may have to ask them for a small favor when you are in a pinch!
The overall takeaway: don't let people take advantage of you and respect your own personal needs. We all know someone who leans on us a bit too much. Establish your boundaries and work from there. Saying "yes" all the time can lead to burnout and overall, stress. We already have enough to worry about, don't freak out about fixing other people's problems.