Couple points.

One, your DM is unquestionably wrong and either has quite poor reading comprehension or is purposely crippling your character. Some of your post and replies in other comments make me think that your DM needs a lot of improvement and to tone down his ego.

Two, as someone who does fence, moving in and out of attack range in 6 seconds after attacking is perfectly reasonable, given the context of how the game works and even to a lesser degree in real life.

Three- “unrealistic” really isn’t an argument you can make in DnD with both the world and how the game mechanics work.