It is a breaking of boundaries. Its pretty much to be expected. Most women will always want a little bit more than they agreed to, never a little bit less. If you don't pick this one up it will slide further.

Don't start by going nuclear because then it becomes about the argument and your will versus hers not the breach of promise that you actually want to talk about. Don't go in "Looks like I will have to dump you if you don't comply". Start off saying "This isn't what we agreed to" and try to move to "So what are you going to do about it?" and then see where she goes. Try to avoid the "well its over if you don't..." Let her see that for herself and choose whether or not to avoid it. In the end you may have to dump her but use the last resort as a last resort or you become the "bad, paranoid, intolerant, controlling guy using emotional manipulation" that the left love to call men, not simply the victim of a broken agreement.

How am I supposed to approach this topic? Is this simply just a value mismatch?

Here's the problem, at a deep deep level.

She is crossing boundaries and you are trying to work out how to enforce those boundaries WITHOUT LOSING HER.

You are TRYING TO MAKE IT WORK and she is subtlely pushing boundaries.

And you are now here trying to get us to help you MAKE IT WORK. You are the one trying to make it work, she's the one pushing things.

The fundamental issue is that you are in a weak position and she is probing to find out how weak.

The reasoning was something along the lines of wanting to meet an old classmate from her city, and that it has been so long.

I actually think it's reasonable, but at the same time it was a clear boundary breach.

Can you reason whta makes for you

reasonable boundary breach

Dont cave in, dont feel the pressure, just plain and simple give your best shot at reasoning what makes it reasonable.

And honestly, it’s starting to feel emotionally draining

Next!

While LTRs aren't going to be all rainbows and sunshines, constantly choosing to stay committed to the same girl, shouldn't be emotionally draining. Any woman worth committing to should add not take away from your life.

Had I adopted this line of thinking in my 20s, it'd have saved me a decade of headaches and heart breaks.

Mattyanon gave a great response for the case of a genuine boundary push.

I'm going to consider the honest ethical case of normal socializing with old acquaintances in her home city which can be perfectly legitimate by a non single woman.

In the big picture, you have to let go of anxiety and control and give people the opportunity to demonstrate trust and commitment worthiness. Too trusting can be a mistake, but the other extreme, too controlling or concerned, doesn't make you a great value proposition from a woman's perspective either.

To help calibrate this internally, imagine your sister was dating a guy and this scenario came up. How would you want the guy to behave towards her?

You break up with her.

Trying to control what women do doesn’t work.

You’re trying to bargain with chips you don’t have because you’re trying to keep a relationship. This is leverage for the female.

She will get away with what she thinks she can get away with, because she knows you won’t leave, or doesn’t care about losing your approval.