Good questions and good concerns, P.T.
If you have time to read, boh to ease your concern and gain some tools to make parenting easier I would like to recommend "Parenting With Love and Logic" and "Parenting Teens With Love and Logic".
One reassuring thing they teach is that if you've had a good relationship with your kids up to about age 10, the teenage years may or may not get a bit hairy, but it will tend to come full circle and be good as adults. That encourages me mightily.
Next, I have to also agree with kscouple. God has your back. "I will contend with him who contends with you, and I will save your children." (Is 49:25) and "All your children shall be taught by the Lord, and great shall be the peace of your children." (Is 54:13) are great promises for parents, and they carry no requirement that there be both a dad and mom in the home.
Having said that, having a dad around is still the best situation and well worth the trouble. If that [roves impossible, then Tlaloc's suggestion could well be better than nothing. However, if you were to allow another man that much respect and authority and involvement in your daughters' lives (and you would have to model those if you wanted your daughters to, as they learn more from our example than our words, right?), then why not just marry him and get the full package?
Bottom line? By all means, get married. It's good for you and them (assuming you choose wisely). But ease up on yourself in the meantime. Sounds like you are doing good, and and Father God is on the job.