When words are sweet, there is a positive impact. Staying constantly cheerful isn't an issue, if only we know how to do it. Sweet words are like flowers, when given to others brings happiness and spread love. Harsh words are like stones that hurt. Harsh words emerge when there is some weakness working within us.
So what do?
Try not saying anything that is not sweet. If a situation does arise where you feel compelled to say something not so good and nice, move away from there. Till you're not sure that you'll not speak nasty words, do not come back to that situation. What works at this time is to try and resolve that point in mind so that you don't speak words that leave a negative impact and make you feel bad later.
It helps, just try!