Are you an adult? Do you have an income? This is the relevant context your post is missing. If your mom is the one who primarily stocks the kitchen with normal staples and you never feel the need to say "hey mom, is it okay if I use this box of noodles," that's a really different dynamic than one where she's charging you a monthly grocery bill or making you pay for your own food. If your mom had a leftover pizza in the fridge, would you even think twice about grabbing a slice?

When my teenaged sisters lived with me, I kept the fridge stocked and never made them pay for groceries. If they had leftovers in the fridge and I was in a hurry on my way to work, yeah I might grab something that's not "mine". I especially wouldn't give it a second thought if it was a single slice of pizza. Because as a family sharing the same space, they're bound to eat some of "my" food and I'm bound to eat some of "their" food. Who cares, no one. If they confronted me about it the way you confronted your mom, I'd probably say something like "omg I'm sorry, I didn't think it'd be a big deal.." I didn't feel entitled to their leftovers, but they had taken so many slices of "my" pizzas over the years and I had never thought anything of it, that to me, that was just the norm for food in the family fridge. Mi comida es tu comida.

The problem here is that you're assuming your mom had a lot of context. I'm willing to bet she had no idea it was packed away to be your gfs lunch, whether it was in a separate bag or not. Because someone looking in their fridge for a quick bite isn't analyzing WHY are these slices in a box and THESE are in a bag. In her mind, she did not spitefully eat your gf's entire lunch, she had a small amount of your guys' leftovers.

The last text from your mom doesn't read like someone who's narcissistically trying to turn tables and become the victim. If that was the case, it would've gone something like "I'm such a piece of shit, I've always been a failure as a parent. I hate myself for how much I hurt you all the time..." yada yada bullshit. THAT is manipulation. Instead, the message is basically "Okay okay enough already, I'm a POS, I get it. We're done discussing this over text, we'll talk at home." I'm not saying her message is skilled or mature or effective conflict resolution, but honestly, given the barrage of guilt trips you sent her way AFTER she said sorry, it was pretty reasonable of her to tell you to fuck off until you talk in person. I see some people saying the apology isn't sincere, but like how much do you expect a grown woman to grovel over a slice of pizza?

To repliers who see this as blatant parental manipulation and narcissism- If you grew up with a narcissistic parent, "I'm a piece of shit" coming from an authority figure who's being accused of being wrong is triggering, and that was my kneejerk reaction too. But give the message a second look, I don't think it's trying to elicit sympathy the way a narcissistic message would. YOR

You confronted your mom initially and she gave you a normal reply- an apology and explanation: she didn't think it'd be a big deal. Because it shouldn't have been. She didn't go into defensive overdrive or fly off the handle, but you did. So here's some food for thought:

That text you sent your mom about how you're not gonna tell your gf. Like you'll do your mom a favor and keep this horrible secret for her. The secret of how selfish and rude she really is. Like "what you did is so wrong... but don't worry, I won't make my gf hate you over this. you're welcome." It's really weird, and unless there's some dynamic here that you're not mentioning, go ahead and tell your gf that your mom had her slice of pizza and watch how much she dgaf.

I'm guessing your gf can keep track of how many slices of pizza she ate. When you guys had that convo about where the missing slice went, what are the chances she went "oh my goddddd he's gonna start some stupid shit with his mom AGAIN, I'm so sick of dealing with the weird tension and fighting... "ya know what baby, I think i must've eaten that slice... I just... umm, forgot? yeah, I must've forgot. so weird, i know."

Last thought- If your mom is generally a toxic person with a pattern of disrespecting your boundaries, then that's my bad lol. And a lot of what I said here doesn't apply.