I feel guilty for being part of cheating - and I feel used because I was led to believe it was going somewhere

I’m struggling to make sense of a situation where I feel both ashamed of my own actions and deeply hurt by how things played out.

This started with someone who had been a close friend for a long time. Our relationship was completely platonic for years. I never pursued her, never saw her romantically, and didn’t even find her physically attractive at first. Over time, emotional closeness grew, boundaries blurred, and eventually we slept together after she initiated, while she was still in a long-term relationship. I know that was wrong, and I take responsibility for my part in that. I feel real guilt about being involved in cheating.

After that happened, I set a clear boundary. I told her we couldn’t sleep together again unless she actually broke up with her boyfriend. I meant it.

Her response wasn’t to pull back, it was to give me hope. She told me she had started looking for a new apartment. She talked about changing her situation. She framed things as “complicated but moving.” That’s when I got strongly attached. Not because of sex, but because I believed she was taking steps to make this real.

Only later did I find out there was another guy in the picture - and not casually. She was emotionally attached to him. She cried over him. She talked about how intense their connection was and how much he affected her. This was all happening while she was still with her boyfriend and while she was getting closer to me.

At the same time, she consistently minimized that other guy to me. She said he “meant nothing,” that it was just physical, that she cared about me far more. She told me she felt safe with me, that I mattered more, that what we had was real.

Her actions reinforced that belief. She gave me very mixed signals: kissing me and then pulling away with “later” or “tomorrow”, holding hands, cuddling, long make-out sessions, joking about future plans, talking about living together, letting other people assume we were together and not correcting them

It felt like couple behavior, not something casual.

Eventually, she started being flaky and when I pushed for clarity, she told me she didn’t feel romantic or sexual attraction toward me and that she needed to focus on herself. She also told me she had started therapy.

That left me feeling like I wasn’t chosen, I was used as a buffer. Someone safe and familiar while she was emotionally hung up on someone else she couldn’t let go of, and while she also stayed with her boyfriend.

Now I’m sitting with two things at once: guilt for crossing a line that went against my values, and a sense of betrayal from having my boundaries acknowledged in words but undermined in actions. I’m trying to understand how a long-standing friendship turned into this, why I got attached only after being given hope of change, and how to process feeling both responsible and misled at the same time.

If anyone has been through something similar - where attachment formed not from chasing, but from being told “things are changing” - I’d appreciate hearing how you made sense of it.