Does having more faith improve cancer outcomes?

There is no evidence for it.

Three meta-analyses published together in the journal Cancer in 2015 pooled hundreds of studies of religion, spirituality and health in cancer patients. They found small positive associations with patient-reported physical wellbeing, mental health and social health, with correlations of roughly 0.15 to 0.20, which the authors themselves described as modest. What those studies measured was how people felt and functioned, not whether their cancer responded to treatment.

One finding is worth knowing if someone has told you to pray harder or attend more often. In the mental health analysis, religious behaviour such as service attendance and frequency of prayer showed no significant association at all. The association came from a sense of meaning and peace, which is not something a person can produce on demand by trying.

Separately, randomized trials of prayer for medical outcomes have not found an effect.

Catholic teaching says something similar in its own terms. The Catechism states plainly that "even the most intense prayers do not always obtain the healing of all illnesses."

If this is being said to you repeatedly, a hospital chaplain can help, and can often address it with a family member more effectively than you can.