I'm personally not nostalgic and prefer variety to repetition (which I can find to be a killer), but when needing comfort, then I want the familiar. The tertiary is supposed to be a stress relief anyway.
See Van Gogh for a good visual idea of Si in an INFP.
I find Si less about seeking the "same" or living in a memory than collecting nuances of experience and instead seeing reality as a "should" concept - what reality should be like. This is actually why Si tends to be negative for INFPs when too heavy - there's too much comparison with the "should be like" and the "what is" or "what is likely to be".
I would give this exercise to simulate this...
Imagine an apple.
Now, apples may have many different appearances, so how you imagined an apple is really your individual perception of what an apple looks like. This is not fanciful nor creative, but it's still a subjective construct of the mind. Likely, you did not even remember a specific apple (especially if you had not looked at one very recently), but instead your brain projected a sort of composite image created by your many experiences of apples.
Si can be like a composite image which produces an idealized reality - how something SHOULD be if you took all the best nuances from specific experiences to produce it. But the Si-dom seems to have a clearer grasp of the details that went into the composite and the way they relate to one another, and they have a more pragmatic vision because of it.
Whereas an INFP may only have the hazy, impressionistic vision which blurs out the impossible combos, and it only becomes crisp with details when something in reality triggers a memory that is rife with meaning to us personally or when something very close to it manifests the ideal reality. When it fails to manifest, then bitterness may ensue...