Most couples who have children end up in a similar state, i.e. the amount of time that you have for your own activities are very limited. For most couples it comes on gradually, you start with one baby and end up with two or more children - where you have leapt out of one life and into another. As such the changes to your life are huge.
I was lucky, my better half and I worked out a way of doing our own things as well as family things. It was not perfect, but it just about worked out. Our children are now 18 and 20 and I am having to relearn how to fill my own time, rather than be child focused.
Have you talked to you partner about this. And I mean talked and discussed it. Is she aware of how unhappy you are? Have you worked out what you want - do you want one day a month of 'your' time - one weekend a month, every other weekend? From the sounds of it there is a large part of your life that you like, but it needs to be adjusted.
Financially you sound like you are two people renting a house together, rather than a couple living together. Have you talked to her about you finances and how it appears (to you) that you are worse off. She may not be aware, or may be aware but just ignoring it. As the higher wage earner she should expect to contribute more, added to the fact that she brought three people into the relationship, in effect you are subsidising her. As such it is not unreasonable to expect you to feel pissed off.
At the root of all this is communication. You need to talk to her, if necessary with some professional help.
