Keeping the first plan when a better one pops up is what makes your yes worth anything.

Something more appealing will almost always appear after you have already said yes, and it will arrive feeling like new information rather than temptation. The worth of your word comes precisely from the times you keep it when keeping it costs you something. If your plans dissolve whenever a better offer shows up, people quietly learn to treat your yes as provisional.

Say yes more slowly, so you are not constantly stuck with commitments you resent. Once given, treat the question as closed and stop comparing. If you genuinely must change it, tell the person early rather than at the last hour.