@nancydrew89 - I have three boys and no girls, and I can absolutely promise you that it is lovely.
I never found out the sex in advance, and each time I was pregnant, we had both a boy's name and a girl's name in mind - it was always the same girl's name, because we really liked it. When I was pregnant with dc3, I really struggled to find a boy's name that I liked (mainly because I had foolishly used my three favourite boys names for the first two boys), but dh came up with a boy's name that felt perfect, instantly. It felt so right, that I think I would have been disappointed if ds3 had actually been dd1, so I can understand a bit of how you are feeling.
In some ways it might be better that you have found out now - you have the rest of your pregnancy to deal with your feelings and move past them.
My boys are 22, 24 and 26 now - two have graduated and left home to start jobs, and the third graduates this year, but has basically moved out because he has a job and a girlfriend (and a busy social life) in the city where he's studying, but I look back at the years of raising boys, and I have so many happy memories.
Some of them are funny - like learning to cope with the amount of Lynx Africa that a teenage boy can spray in one go (so much that you can taste it, three rooms away and on a different floor
), or breaking up a fight, and asking the one who threw the first punch why he'd hit his older brother, only to be told "Because he was looking grumpy, mummy!!" - and surprisingly this hadn't made his brother less grumpy! Or when they are taller than you and get to patronise you whilst getting things down from high shelves for you.
Or there are all the lovely hugs you get from boys - especially when they are taller than you, but still want hugs from their mum.
I love my boys, and I am sure you will love yours too - but you have every right to be feeling a bit disappointed now - your feelings are valid. Let yourself feel the emotions, and then you will be able to move on from them, and enjoy the rest of the pregnancy and your lovely, squidgy new baby!
Good luck.