Treat it like a flashback:
* Remind yourself it is just a memory or idea. It is not happening now.
* Ground yourself in your body, this time, this place.
* Use your senses. Sometimes a strong smell or flavor helps, or a visual image to overwrite the memory.
* Specific to farmemory, concentrate on things unique to this timespan. I use cheap paperback books, because they only exist for a comparatively brief time between rare expensive tomes and everyone reading ebooks so paper ones become something only collectors bother with.
* Soothe emotions with gentle music, a warm bath, a fuzzy blanket, or whatever else feels relaxing to you.
* Remember that historic people taught what they believed not necessarily what was true. Ideas about race, religion, identity, sexuality, morality, justice, anatomy -- these and many other things have evolved much over time. Sometimes looking at a timeline helps.
* You may want to write down what you recalled in as much detail as you can. Not only is this useful for later reference, the process of writing things down often erases the immediacy of them from the front of the mind.