Hi, I'd like to help you with your question.
It sounds like you feel guilty for ending your marriage to your daughter's father. That is actually not a bad thing (though it doesn't feel so good). Your feelings are a sign that you are a good mother and that you care about the effects of a father on your daughter.
There is nothing wrong with the relationship you have now with your boyfriend and the effects of that relationship on your daughter. It sounds like your boyfriend and your daughter have bonded, which is a good sign that she feels safe with him and sees him in a parental role. It also sounds like she has a great relationship with her biological father as well. This gives your daughter two very good role models in her life.
If you chose to remarry her father, consider that it may not work out. The reason you would chose to do it is so your daughter would be with her biological father. But you would not be doing it for yourself or any strong feelings you have for your ex. This is not a good foundation for your relationship. Also, you would be faced with the same issues you had together when you were divorced before. If you feel you could over come them, there might be a chance. But if you would end up divorcing again, your daughter would be greatly affected by that. She would have lost both your boyfriend now and her parent's marriage all over again.
The best option for your daughter's well being would be to make your current relationship work. Once your daughter is older and you can explain things to her about your divorce, let her know you did your best and you care about her. She does not need to know that you felt you did not try every option in your marriage. All she needs to know is that everyone in the situation did the best they could because they love her. This is the best way to help her feel secure and loved.
I hope this helps,