I paid for my trip 2 months ago and am now on chemotheapy treatments. Will travel insurance cover this if I need to cancel?

Unfortunately, travel insurance covers cancellation for ”unforeseen” reasons as listed in the policy. To obtain a waiver for the pre-existing conditions, you must purchase within a short time after the first payment (7 -30 days depending on the policy) or prior to the final trip payment for a few policies. In the event you had to cancel as a result of the treatments, you would not be covered.