So torn 😢😢😢

So here's my story. I met my husband when I was 17. He was my first love and first boyfriend ever. We went to HS and college together. Right after I graduated from college, I broke up with him because I felt we needed time apart. Well, I was in a vulnerable stage and started dating a close friend of 12 yrs a few weeks later. 4 months later, I found out I was pregnant. My boyfriend left me and my husband stepped back into the picture as a friend. He went to appointments with me, made sure i ate and he was my biggest support system. When my daughter was 5 months, my husband and I decided to give our relationship another try.  We since have had 2 more daughters, got married and are now expecting a 4th child. My oldest daughter is almost 6. Her biological father is not in her life AT ALL!!! My husband is the only father she knows. At times, because of the love my husband has for her, I forget she's not biologically his! When and how would be best way and time to explain to her? Thanks in advance for your help!