DSS, it's hard to let go of something once you have become so focused on it, I know just saying it dosn't mean you can do it.
I wonder why you are so disastisfied? You mention your boys are hard work, maybe it's your experience so far with them that you are struggling with and have an idea that it would be esaier/better/happier with a girl? If there is some truth in this, then it would be better to think about how you can become more satisfied and work towards making things easier and more enjoyable. The more you can enjoy your boys the less you will feel you are mssing out.
I also think you have to start to accept the life you have and stop focusing and dwelling on the things you don't have. Stop the thoughts about what you are missing and daydreaming about what it could have been like and start focusing your thinking on the good things about what you do have.
Are your boys affcetionate? think about the special bond mothers have with boys, how often they are more vulnerable and need you more, about what simple and striaght forward ceatures they are, about what funny strapping young men they will be who will compare all women against you and how you will fiercly defend them against all potential heartache, how as men they will want to protect their Mum and take contraol and sort things out for you etc etc (others feel free to add to this list).
there are things you won't get to do if you don't have a daughter, but let them go and try to focus more on all the wonderful things you do have.
become more concious of your thougts, try to stop the dwelling, it will be hard at first but if you can start to change how you think about your situation you will start to change how you feel about it (ask any CBT counsellor) and will find you can become happy with what you have.
If this sounds flippant, it is certainly not meant to, I know how hard it must be