My H had a similar family situation and he too believes that as long as he is part of S's life then he is a better dad than his. I want to shake him and as long as I am shaking him I can shake your H too. Part of being a good dad, a great dad if you will, is being part of your family, being part of a family where both parents are living in the same house, together. Not only is it an example of commitment, but the financial benefits to marriage far outweigh those of being single. As a kid I hated shuffling back and forth between two parents. Divorce sucks for kids. We live in a community where family is key and I hate going to fairs and events with just me and my son. Every one of my sibliings is married (and quite commited to weathering the storms).
I heard something really powerful today from Joel Osteen. He said "everything is subject to change. Your change might just be 72 hours away." Every time I feel like giving up I am going to think of that.