Yeah, you need to make Lena cheat once. Just don't be a serial cheater.
What I did was make Lena cheat on Ian with Jack, stop Jack's path then and there, after the incident. Commit to the relationship seriously in EP12 in her bedroom.
From Ian's side, don't get aggressive. Tell her that you still love her, have a romantic kiss under the moon, get back to the room, punish Lena and during the scene when Lena asks "Do you really love me"?, say yes.
Congrats!!! She's now totally hooked on Ian.
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"I need to" only pops up when she's a sub to Ian.