Just because you want something done right now, tomorrow, or next Tuesday, doesn't mean it's going to happen in that timeframe.  Same with client requests.  Clients almost always want something done sooner than is realistic.  So which person are you? The person that promises delivery on that unrealistic deadline because you're afraid to say no?  Or the person that is confident and real enough to just say, "No...it can't be done that fast. But this is what I can do."? 

I've become the latter but was once the former.  Mainly for all of the reasons you can imagine. I was afraid to let clients down.  Afraid they would think I was incapable.  Afraid of losing my client to someone who could move faster. But by saying "yes" to deadlines I couldn't meet, I was actualizing all my fears.   The focus moved from my capabilities to my inability to meet a deadline I had agreed to, regardless of how well I completed the project.

Bottom line...don't be scared and stress yourself out by agreeing to do something in a timeframe you can't meet.  Confidently speak your truth and your clients will respect you for it in the long run.  You're not going to lose them.  More importantly, you're not going to disappoint them, just increase their confidence in you when you're able to deliver on time, as promised. And best of all, you won't play yourself by trying to meet unrealistic deadlines.