just tell him the truth short term pain long term gain
This is a truth that most people who are getting into a relationship with someone with children need to understand. His/her children will always come first, sometimes his ex wife/girlfriend will be around. But love is selfish and you want your partner all to yourself, this is natural. You need to tell him how you feel, he cant read your mind. <br />
You will need to be tough but you have one life, one life to live the way you want too. If you're not happy, then fix it. It's the only way. I would suggest talking it over with the father before bring the kids into the fold. No heart has ever broken even, someone is always hurt. But it's better than living a life of endless misery.
That kind of depends on where you live. Is it your place, his or both of yours. If it is his or both of yours, just move out. If it is yours, it will be tougher, asking him and his kids to leave.
You can't break up without creating havoc in people's lives.