Adding flavor after beer is finished

So, I brewed up an ESB about a month ago using my own recipe, kegged it a little over a week ago, and just got a chance to try it last night. It tastes awful! There is little to no sweetness at all; just terrible UK hop bitterness/flavor. And I started thinking that maybe I could vent the keg, open it up, drop about 1 1/2 pounds of unmilled Crystal 50 in there (in a hop bag), and let it sit for a week to counter some of that bitterness. Has anyone done this before and with what results?

Also, I am posting my recipe below for criticism since every time I've brewed using my own recipe, I've been dissapointed. Thanks.

9 lbs Maris Otter
1 lb Cara-pils
12 oz Munich
12oz Crystal 20

Mashed for 60 min @ 152-ish

2 oz Willamette pellets @ 60 min
1 oz Fuggles leaf @ 20 min
.5 oz Fuggles leaf @ 5 min

London ESB Ale yeast

OG - 1.061
FG - 1.015

(I'm looking at this thinking either I put too much cara-pils in, not enough crystal, or somehow I allowed my FG to get too low) ...?

Adding unfermented grain to a batch that's done is asking for a lactobacillius infection. You'd need to mash or steep the crystal, pitch yeats to it, let it ferment, then blend the two.

Raw grain is covered with lactobacillus and will infect your beer. Do you want a sweaty goaty flavor?

Your recipe is not too unlike mine in ratios. I use:
9lbs pale malt
1 pound Crystal 40L
1/2 lb Victory

My hop schedule is similar to yours but using East Kent Goldings and Bramling Cross.

Mine is wonderful, so I can't explain yours.

Wow, I forgot all about sanitization. Good thing I brought it up. How strong of a mini-beer do you think I should make to blend with?

Also, this could be completely wrong, but I feel like I heard somewhere that a purely Crystal mash would be unfermentable. If that is true then I could just boil it, chill it and blend it with the other beer right?

No crystal has available extract just like your base grain. If you didn't ferment it separate the yeast in your beer will.

Don't drop anything in your beer. The bitterness will mellow as your beer ages. Wait a couple of weeks and try again.

Your final gravity sounds fine.
You have lots of bittering going on there.
You also have lots of dextrin malts going on there (carapils & crystal).

If your beer is still too bitter after a couple of weeks, take note of your BU:GU ratio ("bittering units" to "gravity units").
It is around 1:1 (1.0) and you may want to lower it to closer to .5 or .6 in the next batch, and then go from there on future batches.

You may also want to lower your carapils + crystal malt additions to 1 lb max in a 5 gallon batch.

You may also want to look at your water. Does it have chlorine? Is it hard or soft? Do you need to make adjustments?

Not sure which malts are dextrin and which ones aren't, but are you suggesting I use something other than crystal to sweeten this up? The water is water I've used many times before for bitter beers and never gave me issues.

Cara-pils is a dextrine and Crystal also contains dextrine as well as lamentable sugars. Worticia is recommending you lower the total amount of such to no more than 1 lb.

You could drop the Cara-pils and just use crystal.

Of course this will make it drier and may make the hops stand out more. But ESB is supposed to be hoppy. Like Worticia said, let it ride and see how it smooth out. In the future check your sulfate to chloride ration in your water as well.

I know this is another question, if your looking to add sweetness to it what about lactose, doesn't that add sweetness?

Lactose adds a little but of sweetness, but you have to get pretty concentrated (over 1/2 lb in 5 gals) for it to really come through. In to that point I only get a thickening, smoothing effect. In an over-attenuated blonde ale, 2 oz lactose and 2 oz maltodextrin gave back body and mouthfeel, but didn't help the thin taste.