Mercury goes retrograde usually three or so times per year and it lasts three weeks or so, but it can be shorter. If Mercury retrograde "caused some stammering" every person on earth born at that time would stammer. There are a lot of charts out there and it is impossible to say that everyone that has this in his or her chart has this or that problem or this or that good fortune. Astrology does not work that way. I have Mercury retrograde and I don't stammer. I may have other problems that can be attributed to Mercury Rx or other things or just being human, but that isn't one of them.
I have two examples that might be of interest although, as you correctly point out, it is the person that matters not the difficulties. King Charles I of England may have had a stammering problem. His Mercury was direct, however it was combust, in Sagittarius, the sign of its detriment. Still this is not enough. Millions of people have Mercury in Sagittarius combust.
George W. Bush may not stammer, but he gets his words all messed up at times. His Mercury is direct in Leo on the ASC. What's going on? Both cases have something in common. Both men have strong choleric temperaments. Choler is the fire temperament and impatience is one of the characteristics. Now combine impatience with an active mind in stressful situations and struggling to get the words out as quickly as they come into one's head and mistakes, stammering, what have you, might occur. Every situation does not manifest the same way for everyone.
Charles was King during the English Civil War and this is no place to get into the details, but Charles took personal offense at the idea of Parliament having any controls over the king. He was a strong believer in the Divine Right to Rule. In the current vernacular he "couldn't believe" what was happening to him and the emotional stress caused by his temperament and his situation must have been enormous (Civil Wars will do that to a leader) plus the choler in him trying to get out, and his debilitated Mercury may, I say may, have contributed to his speech problem. Charles was eventually beheaded a problem more pressing than his speech.
George W. Bush is highly choleric and with his Mercury in a fixed sign on the ASC where everyone can see it, his words come out all wrong at times. Think of Mercury as the traffic cop of the words. They are coming at him, mentally, so fast and furious (a good choleric word) he cant control them Something has to give.
The point is we cannot look at a single placement or condition and come to a conclusion because if it were that simple, it would apply to everyone the same way. We know this isn't true.
One more point. Temperament, as shown by the chart, is not necessarily the same as personality. Although we can't be absolutely certain, in the above examples, Charles probably had a choleric type personality, George Bush does not. It is more subtle at times. You mentioned you have to regain your composure once in a while. This sounds like choler to me. I like the way you face the problem head on (that sounds like choler, too). You'll be fine.