What do when your gf is growing distant

We're in a ltr ldr poly. Yes she started dating a new guy but we agreed it wouldn't be a reason to spend less time together.

Except that's exactly what she's doing.

It feels more like an affair than true poly.

I told her to stop dating the guy, she doesn't want to.

And i've been with her for 4 years, love of my life, we've been through a lot, so i'll try everything possible before i leave her of course.

I've been confronting her about it, she just says she's been busy. Huh, not too busy to spend 8 hours a day with her new bf apparently. Honestly this kind of lying and weaseling pisses me the fuck off.

If she doesn't love me anymore i deserve to know. Because ignoring your first boyfriend hoping he goes away or something is fucking childish. It's like she's waiting me to break up so she doesn't feel bad about her doing the break-up. Weak and pathetic really.

I want to corner her into speaking up so we can decide if she will commit to spend more time with me, or to tell me the truth so i know if i should pull myself or what.