Just to rule some stuff out.

When you text other people do they receive your texts?

Is your partner receiving texts from other people?

Hi there! Thanks for responding. Yes, other people are receiving my texts, and my partner is able to send and receive texts from other people.

Any setting you can think of that you changed recently?

Did she recently switch to an Android device from an iPhone? If so, she will need to deregister iMessage from her phone. https://selfsolve.apple.com/deregister-imessage

Hi everyone! Thanks for your messages.

I haven’t changed any settings on my phone recently. This only started yesterday, and I haven’t made any changes, nor had any updates.

She’s had an Android for the last few years, and we’ve had no problems up to yesterday.

Could this be an Android/iPhone issue? I’m getting texts from other iPhone users. 

Have you tried deleting her and adding her back to your contact list? You can then send a new message to see if it goes through. 

So the issue resolved itself yesterday - I still don’t know for sure what caused it. The only thing I can point to as a possible reason (and perhaps someone can weigh in if they know for sure) is a VPN service I had put it on my iPhone and iPad. I deleted it off both, and things were fine a few hours later.

I’m still not 100% sure this was the issue. I had this service for almost two months and there had been no issues whatsoever, and I made no changes to the service. Maybe her Android system or the carrier (Virgin Mobile) eventually detected the VPN and blocked my texts? I’m no expert by any means, but I can’t offer up any other explanation.

At any rate, thanks to everyone who offered help. I appreciate it.

Glad to hear that it has been sorted out.

It should not have affected your SMS text messages. However, you could be right if you were sending MMS messages over the phone. Those require your cellular data connection and may have been affected by the VPN service.