Must I break my fast now, knowing that the blood has become thick and dark, and that what preceded was considered istihadah? And must I make up the days I fasted?

You are experiencing metrorrhagia (istihada), and you must make up the days you previously fasted, because a menstruating woman's fast is not valid.

If you have a prior accustomed period, then you should abstain for the duration of your habit, then perform ghusl, and consider any excess as istihada, for which you perform wudu for each prayer.

If you do not have a prior accustomed period, then rely on discernment: whatever has the characteristics of menstrual blood is menstruation, and anything else is istihada. After it, you perform ghusl and then perform wudu for each prayer.

If the bleeding and the intervening purity exceed fifteen days, then act according to discernment, provided that the discerned period of menstruation is not less than one day and one night and does not exceed fifteen days.

However, if the duration of the bleeding does not exceed two weeks, then all of it is considered menstruation.