I've been through challenges in this area, some of which I'll mention as time goes on. So I can definitely relate to some of these experiences. The goal in all this is to become better dads, not bitter dads, from these experiences. It's easier said than done, I know, but it's ultimately better for you when you do so.
We often hear the term "deadbeat dad". But I encourage you to become an "upbeat dad", one who's enthusiastic about being "dad" and who's involved in the lives of his child(ren). That's a reward unlike any other - helping to shape and mold the life of people who will be the future of our world.
I hope that as you read, you'll be encouraged and inspired. And that your enthusiasm will never waver. Be "upbeat" about the awesome role that you play!