Pardun, is no-one gonna talk about the "long-time" lover Matsuri's mentioned??? Is it just a lie or is she referring to Harumin?
That's the big question, but I don't think at this point that anybody has much evidence to answer it one way or another.
Possibilities:
1) It's someone we don't know and have never seen. Very highly unlikely--if there were such a person, why wouldn't she think of them when she's so bored (and playing romance games)?
The rest of the possibilities aren't "real" relationships, but some form of wishful thinking/speaking figuratively on Matsuri's part.
2) It's Yuzu. We've had indications early on that she had a crush on Yuzu, but overall she seems to see her as more of a Big Sis than as a love object.
3) It's Harumin. There have been plenty of indications that Matsuri has a significant romantic interest in Harumin, but they can hardly be said to be "in a relationship."
4) It's nobody, and she's flat out lying, for any number of possible reasons: to hide her lack of actual romantic experience, to hide her interest in Harumin, etc.
5) The fact that she says she "always has been" in this supposed relationship suggests it might even be about something from before we first met her, which could even be her family (i.e., taking care of her little brother), although that seems excessively corny and sentimental, given what we know about Matsuri.
Since all we can do is wait for more evidence, it may primarily be bait to keep readers speculating about what will happen with Matsuri.
I assumed she meant her experience with dating simulators, like the one she's playing at the beginning of the chapter.