I got cheated on but decided to stay

Problem: My partner (21) and I (22) have been dating for 5 years now. She cheated on me 4 years ago. And I question myself whether I'm stupid or not na nagstay pa ako. I often see posts kasi about cheating na, "once a cheater, always a cheater." Ayoko naman sana i-regret na makipagbreak ako. Naaapektuhan kasi ako to the point na parang jinajudge na rin ako ng friends ko for what I did, nagjojoke rin sa GCs namin na backhanded on how stupid I am, and it extends to my partner who, ever since that day, has been mentally torturing herself for what she did kasi ino-open ko rin sa kanya yung frustrations ko.

What I've tried: I tried defending my partner sa friends ko. I often shrug off what I see sa social media about cheating. I am also moving on sa nangyari but everything flashes back talaga kapag ayan ang topic.

Advice I need: What do I need to do? I need someone to tell me kung tanga ba talaga ako.

Additional Information: We started off online, I met her online. We were friends at first, became best friends, then to lovers. And btw, we're wlw.

I admit na we were kind of still young and immature pa, given that we were 17 palang noon. Nonetheless, nagustuhan ko siya since she was the only one na nakilala kong may sense kausap, kamatch ko ng energy, and not to mention na malaki talaga pangarap niya despite the hurdles she was facing, so I got inspired na rin. I won't tell what she went through, but she's been through a lot (talagang traumatic) at such a young age, and I'm proud of her for overcoming those dark days.

It was during her friend's birthday somewhere out of their hometown nung nagyari lahat. LDR pa kami that time, we haven't met pa in person but we talked and vidcalled a lot.

Now, ever since, hindi talaga umiinom yung partner ko. But that night was an exception. And I guess she didn't know her limits pa (yan yung pinanghahawakan ko na reason). She said she couldn't remember anything, and if meron man daw, it was so vague at paputol-putol. Based sa kwento niya, it was more on about touching. I believed her since anything about NSFW stuff ay ilag talaga siya when it comes to others (ik this since mga friends niya ay mej may pagkarawr and I have access to her accounts). It's really early in the morning, with lots of miscalls, the next day it happened na nagconfess siya sakin and was crying so hard. Nakikipagbreak siya sakin noon but ako rin, makulit talaga, I couldn't. Hindi ko talaga kaya. I don't think I'd be able to meet someone as genuine, smart, and in the same wavelength sa akin as her.

I acknowledge na forever na andyan yung scar, and the day na I decided to stay, that's also when I accepted what my partner did and forgave her. She also promised na she'll make it up to me until the day she dies, which she does consistently. Ramdam ko yun (without the jowa bias), talagang hindi na siya umiinom kahit simula nung nacollege na kami. She even transferred to my univ kahit na madelay siya, not wholly because for the sake of being with me but she also believes na she'll thrive dito (from a provincial private uni to UP).