If I assume the later days (8,9,10) will turn out to be Isteehaza would it be correct to have a ghusl after 7 days and start performing salaat even if I am bleeding?

Every month during my Haiz period I have been bleeding for 13 to 14 days continuously.The last haiz habit I experienced lasted for 7 days.To date, I have been counting the first 7 days as Haiz and the rest as Istehaaza. My worry is that since the month of Ramadhaan is coming, if this condition continues I will have to miss the fasts of the later days (8,9,10) of my bleeding because I will not be sure whether I have finished my Haiz or whether the later days of the cycle will turn out to be Istehaaza like previous months. At the moment I have not been assuming and I have been making qadha of my missed prayers.