What is the way to combine my desire for marriage and family stability with maintaining custody of my daughter from her ex-husband, who may demand her custody when she reaches the age of seven, knowing that my financial capabilities are limited?

A mother who perseveres in raising her daughter will be greatly rewarded, for in the Hadith (definition): "Whoever is afflicted with anything from daughters, and treats them well, they will be a shield for him from the Fire." If your soul longs for men, and the daughter will receive proper care from her father, then marriage is more befitting to safeguard yourself from the forbidden, and marriage may even become obligatory if you fear falling into the forbidden. The daughter will be in trustworthy hands with her father. And your mother has a greater right to her custody after you. Seek Allah's guidance through Istikhara for what is best for you.