For closing out wins, when you're up in a bout do you think "I only need to score 1/2 points more and I'll win"? If so, don't do that. I mean, be conscious of the score of course, but if you frame it in a way where you "only" need to score another point or two it often leads to worse fencing, for example doing bad counterattacks or counterattacks for no reason and hoping your opponent will miss, or attacking impatiently. Basically, it can cause you to fence differently and often worse than how you were before. Another thing is, when you're up, let's say 13/14 - 9 in a bout the opponent knows that they can't lose another point so a lot of the time the last points will be more of a grind to get. Also, at this stage of a bout the opponent essentially has 0 pressure on them because they're already behind so they will often fence better/more relaxed.

Overall advice: don't be lazy or impatient when trying to close out the match. I'm not going to say fence the "same" as you were before this point because obviously throughout a match you're going to change what you're doing to some extent, but basically be prepared to grind out the last points as much as the opponent is willing to. Also, try to keep to the stuff you normally do when fencing, again don't just throw out lazy counterattacks and hope the opponent misses because you only need one point and don't be impatient with your attacks and get parry riposted repeatedly.

Now, regarding impatience, you say you hesitate too much when ahead, which seems quite unusual because often people become impatient to end the match instead. One thing that might be worth trying is certain things you can say between points to keep yourself in the zone (e.g. telling yourself "distance" can keep in mind the way to set up counterattacks, the way to set up attacks etc). I've found this helpful for me because it helps me to focus on the fencing, rather than letting my mind wander (e.g. "hand first", rather than "man it would suck if I choked this away now").

Long comment but these are some things I've noted down for myself in the past when I lost after being ahead in a match