The reason INFJ and INFP are not actually similar and so you can't be "sometimes one and sometimes the other" is because of this:
INFJ is mostly concerned with gathering information, processing it inside on an unconscious level, and using it to find patterns and form ideas. When they do use feelings, they tend to focus outward and apply what they know/feel from their observations to other people. This is why they are known as the Counselor, because they understand people in a very objective way. They are good at seeing patterns that others don't see, sorting them out, and naming them. They are usually not overly emotional or empathetic in a traditional sense. INFJ can actually sometimes type as a T type because they can be quite clinical and detached. When they do get overwhelmed by other people's feelings it is because they can't effectively differentiate between their own and other people's, i.e., literally feeling other's feelings or being "infected" by emotions they pick up intuitively from their environment (other people). Thus, the INFJ blind spot is a tendency to over-trust their ability to understand their own feelings.
INFP is mostly concerned with feeling their own feelings, establishing a good and authentic value system, and applying their own view of the world onto what happens in their life. Everything they do, say, feel, and believe is sorted and filtered through their own experiences, their own feelings, and what they believe is - or should be- important and true. They can become easily overwhelmed by other people's feelings insofar as they experience other people's feelings **as they would imagine it would feel if they were experiencing the feelings themselves** which is ever-so-slightly different than how the INFJ feels empathy. It is actually much more common for an INFP to be overwhelmed by guilt or shame or sadness than it is for an INFJ to be. They have a tendency to make up scenarios about what other people's reactions mean based on their own feelings, partly because their use of intuition gives them all the ideas. So they are creative-minded and able to imagine many possibilities, but ultimately they tend to land on the one that resonates most closely with their own way of being. Thus, the INFP blind spot is a tendency to over-trust their ability to understand other people's feelings.
Side note: Because both are deeply empathetic, both make very good counselors, and there are many therapists out there who are INFP or INFJ. But counseling is actually harder on the INFP because they care for people by *imagining how the other person must feel*, (which feels real to them, because they're all feelings). They then tend to hold on to the fear or sadness or anxiety of the feeling as if it's their own, and feel badly as a result. So it's more exhausting for INFPs to sit with the pain of others than it is for INFJ. INFJ objectively *feels* the other person's feelings to some extent, but because they are not predominantly feelings people, they are more *interested* in others than they are actively *caring* about them. So, INFJ is usually able to extricate the feelings and leave them behind. Because of this distinction, a) you can see that they are very different archetypes, and b) they are called the "Counselor" type (INFJ) and the "Healer" type (INFP). The INFJ helps by seeing and naming and sorting the pain of the other, and the INFP helps by sitting with and reflecting and feeling the pain of the other.