Have you ever heard this quote? Don't make promises you can't keep.
It is okay to say no and you can't say yes to everybody all the time. Try saying “I can't do that right now but if things change I will let you know”. See that way you are not saying no and you're not saying yes but you're still coming across as being proactive and a team player.
Let’s talk about personal development for a moment. I heard a really good analogy on this to identify what you can do and whether you should make promises to do something whilst at the same time making sure that you're actually challenging your ability to do something and growing .
Think of it in terms of a fried egg.
The yolk in the middle is what you can do easily and you are comfortable at doing a fantastic job.
The egg white is there area where you're not so sure, but to take on an activity in that area would definitely help you grow and stretch your abilities. This is the sweet spot for failing and learning. Failing is super ok here.
Anything outside of the egg, you need to identify that this is a higher risk and you're likely to fail and disappoint people. Ever extending into these are the tasks or opportunities in life that you should to straight out say no to and when you do this you create a good scenario where everybody knows were they stand, there's no false promises and hopefully nobody gets disappointed. This happened to me when I took an opportunity I was not ready for and it took a long time to recover.
Another similar example to this is what I've heard from Warren Buffett when he talks about the sphere of competence. You should understand what you can do with in a certain sphere and anything outside that sphere you should avoid getting into.
Listen to your gut, talk it over with someone who has been there and done that. Push the limit but stay true to yourself. Keep the failures small and frequent and keep the momentum going.