How would you say "same thing" in Latin? As in the sentence: If only you were feeling the same thing that I feel for you?
This is best expressed with the neuter pronoun idem, "same", in Latin. For example:
Utinam idem sentires ac ipsa/ipse sentio!
If only you felt the same as I (fem/masc) feel!
Alternatively, you could use the plural eadem, "same things". The difference in meaning is not huge, and Latin tends to favour plurals here more than English.
Latin doesn't usually use a noun for "thing" the same way English and many other languages do. Instead, a pronoun or an adjective in neuter is used.
I assume you were only after an expression for "same thing" here. If you want to find a good way to phrase the "for you" in "what I feel for you" in Latin, I recommend asking a new question.
I want to pick up on Joonas' answer, and just provide the sentence with the literal translation for "same thing":
Utinam sentires eandem rem quam sentio tibi
Whilst this might not be very common or appropriate for the particular context, eadem res is a valid expression in Roman Law (see top of page here).