You're Dad, Not Mom and Dad
Finding yourself suddenly a single parent will almost inevitably lead you to this simple conclusion: You need to "replace mom". It's perfectly natural to want to give your kids a female presence in their lives. It's also perfectly natural to feel terrified at the prospect of raising your kids, especially your daughters, on your own.
Getting into romantic relationships obviously isn't out of bounds when you're a single dad, especially when someone special comes along. What you should avoid is succumbing to the urge to base a relationship on your desire for some help with parenting. You should also avoid taking on the role of both mother and father. You're a father and can offer only what a father can offer.
Look around and you'll find there are female figures in your life that can fulfill the void left by the absence of a mom. If you lack female relatives who can step in and help out, organizations like Big Brothers and Big Sisters can help. Just remember not to be hard on yourself for not being mom and dad. No one expects you to be.