Be careful with your words, because once you have said those words they can only be forgiven and not forgotten.

One of the best attributes of us humans is the ability to communicate.

and it is this innate ability that is to used judiciously in a relationship. And by that i mean all kinds of relationships.
An interesting corollary is that we can also communicate our thoughts in real time; we do not need to plan what we’re going to say before we say it. 

This has both advantages and disadvantages.

Advantages are obvious because our brain is trained in a way to react, respond to all situations that are known to us not just when we are awake but also in the unconscious and subconscious state.

Disadvantages are also equally apparent , for all those who do think or regret about what they have spoken either just immediately or after a while.Well the good thing here however is the fact they at least realize their mistake compared to many others who haven,t even reached that stage of realization.

On one hand, this innate ability is often the source of consternation when what we say on the spur of the moment is something we later wish we had either not said, or had said differently.It, happens to each of us. The trick is to remember when. 

Typically, this happens when we are responding in stressful situations, or during confrontation, although it can happen at any time. Recognizing that we do not always say what we would like to communicate is an important realization.

The goal i think is to be aware of when to talk naturally and fluidly and when to think before we speak and when not to speak at all.

Take note of the circumstances when what you say is not what you, upon introspection, would have wanted to say. What we must analyse is, does it happen mostly with one particular person or a particular group of people ? Is it most often in arguments or debates? Is it when you’re "on the spot" and feel pressed to supply information?

If one tries ,one will find a pattern, it will most definitely help us analyse ourselves and the situations which force or coerce us into getting into them and maybe regretting them later.

After you determine what circumstances might be most likely to produce this unwanted effect, try to be very observant about when those conditions appear to be manifesting. The more skilled you become at recognizing this, the better
you will be at changing your approach.

It is this awareness that will help you in the longer run and give you  peace of mind. Once after introspection you will surely reach a stage when you know you’re in one of "those" situations. The goal now is for you to process information.
 Often when we respond in a less than appropriate way, it’s because we haven't fully comprehend what was being said. This is the time to sit back and listen to what’s going on around you. Don’t start focusing on what you’re going to say; just absorb. Your mind will process this information in the background.Though this might sound easier said than done , it will most definitely bring in a change you will never regret.

Who is speaking and how do they communicate? Some people are very literal and some people use examples. Some people use a lot of facial expression and body language to augment their conversation, whereas others rely on complex verbiage. How people convey information is a very good indicator of how they best absorb information.
Not just one, but consider your options. There are many different ways to say things. and your goal here is to find the best way to convey what you want to say in a way that has a positive impact. Communication is primarily a function of the recipient so you have to communicate based on the listener.

Nobody is perfect , but its just an endeavour to reach a state of calm for yourself and people around you.