This is going to be long and quite biased, and I do not know if this will help you.
My first ex broke up with me to immediately date a girl in my friend group. It made me question my self worth back then. I did forget a lot about it, but all I remember is pity. Pity for the girl he replaced me with. If he did that to me, what stops him from doing it to her too? I wanted to be wrong. Years later, I found out that he cheated on her with two other girls, and he almost got one of the side chicks pregnant.
Recently, I had a complicated relationship with my ex that ended drastically. We agreed to keep in contact after breaking up but I made a deal with him: if one of us gets in a romantic relationship, we should let the other know, and cut off contact. He had two chances to tell me that he has a girlfriend already. I asked him if I can still contact him, and he said yes. However, when I was at my lowest, most desperate and most vulnerable state, begging to meet with him because I needed human interaction before breaking down mentally, he dropped the bomb and told me he has a girlfriend. That his girlfriend knows he's still talking to me but he promised her he wouldn't meet up with me, even though he told me he can meet up with me if it was an emergency. His girlfriend also told him I wasn't his problem anymore. He ended up blocking me anyway.
Sorry for the long story.
Was I hurt by how easily I was discarded after giving them my everything? Yes
Did I regret giving them my everything? No.
I did all that I can. I tried everything I could. I have nothing to regret for. I have no what ifs. I may have a lot of "I should've done this and that"s but I did the best that I could with what I was given. And if they decided to discard that easily, that's on them. It's okay to not be ready for another relationship. It's not a measurement of your worth nor does it say anything about you. Personally, I do not want to damage another person knowing I myself am still not well. You're right about the cruel, casual hurt she did to you. No amount of excuses will be enough to justify cheating. When it comes to trusting people, I would not say that you will find someone that you can trust in the future. However, you already met someone who cannot be trusted. You might already know some signs on who to not trust. It might be a long, painful process to find someone you can trust again, but with what you've experienced, you are not going blind this time.