Don't justify your 'no'
As crazy as it may sound, you can say no without any justification.
Like many of us out there cursed with being a people-pleaser, I have yet to master the art of saying no. If someone asks to hang out, if someone asks for a favour, it is difficult to refuse them. Yes and no are both short, monosyllabic words, but the former always seems the most favourable approach.
Not knowing how to say no can stem from a variety of factors. Often, humans are scared to not reciprocate (since not obliging may threaten our connections with other people).
The fear of saying no also stems from the urge to avoid conflicts, or confrontation. Another reason that why people tend to worry about saying no is because they don’t want to disappoint others, or hurt their feelings.Devrupa Rakshit, The Swaddle
I've been there. Unable to say no because I don't want a conflict to arise. Unable to say no because I don't want someone to think that I don't want to hang out with them.
When does it become easier to say no? I find if I do have a very justifiable excuse for missing out on an event or for not having the time to do a favour, then I feel less guilty. They will understand me saying no because if they were me, they would also have to say no. I'm in a position where the only possible answer is no.
But now that my studies are getting very intense, I am learning that sometimes you do not have a valid reason. Sometimes life gets so intense and crazy, and you may not necessarily have something on when someone asks if you are free (so technically, you are free), but missing out on the study time would be stressful. Or often, you feel that you will sound crazy if you suggest you can't attend something because you need time to yourself.
I am teaching myself that it is okay to say no, and there does not need to be some justifiable reason. Perhaps too, feeling burnt out and needing to study is a justifiable reason, and if someone doesn't get that... well, that is on them.
There is a certain power in saying no that can help you learn to set boundaries and manage your time. It can help you deal with conflict. You can simply say, no, that won't work for me, and the people around you should be able to understand. It is important to look out for your own mental health and wellbeing, and that can be done in a way that does not upset the people around you by causing conflict. It is okay to say no, and sometimes, that is the best choice.