The feeling is uneven. It lasts if you keep adjusting, not if you wait to match.
The two of you
Pisces. You feel real when you do not have to harden yet. People see the softness, and someone who understands before the explanation.
Libra. You feel real when the air between people is fair. People see the ease, and the one who makes it pleasant.
What wears it out
A demand to be definite before you are ready drains you. So does a hard room with nowhere to retreat.
Being forced to decide while everyone else stays nice drains you. So does a peace that costs you.
Pisces feels it first and explains later and adapts quickly, and may not finish. Libra needs to talk before it feels real and starts things.
Nothing in the two signs locks on its own. Some days you understand each other. Some days you are solving different problems in the same room. Together, it shows up as sex and what they cannot take lightly. The other way around, it shows up as the work of an ordinary day.
This is the answer people want before anyone knows a birth time. Two real people have more than a sun sign. If it matters, cast both charts.