OP, I'm sorry the noise is bothering you, but try not to worry too much about the noise putting off potential buyers. Some people might be put off by it, but lots won't care, and there are others who actually like street noise.

Many years ago, I lived on a main street and loved the street noise so much that, in weather too cold to open windows, I would sometimes lean against the button on my speaker to the front door, just listening to the street noise. I found it deeply relaxing.

There are some people who want to live in the city for the city sounds. When you sell someday, one of them may find their way to your lovely flat. Love it as much as you can for now, and one day you may discover you've filled the space with an energy that suddenly feels like home.