Brewing short has upset the balance between hop bitterness and residual sweetness. It will probably mellow in time. Most fermenting wort taste more bitter than the resultant beer anyway during fermentation so I would leave it and see.
I had a bitter which I had not hopped enough so I added isomerised hop liquid, I calculated what I had in the beer and what I needed and put it in. Needless to say it was to much and because it was a l=synthetic liquid it was quite harsh. However the point to this story is that now a year latter itr is a wonderful beer well rounded. Only got 4 bottles left wish I had the original 100 left which I painfully struggled through. :lol: :lol:
The only thing which would counter the bitterness is some sweetness in the form of a non fermentable sugars, but i would really leave it till it is bottled or kegged and then try, you will probably be surprised, if not leave it longer. :thumb: