"No" is a complete sentence. For some people they might say something like "I don't feel ready" and it is an invitation for a negotiation, but if it's not true for you, just be firm and say "well I understand you want it, but I don't right now, and honestly I don't need to justify it to you". And that gives the other person a chance to take or leave your boundary. And if they want to leave, then you didn't have to get invested in them.

My life has improved immeasurably since I started setting firm boundaries. For me it was with my parents. As well as some people who wanted to be my friends but wanted more than I could give.