Dude... . it's a setup. You were put in an impossible situation where she got you to agree to an "excuse".  :)o you really think it just happened that way? No. I've experienced that with my ex wife. And of course, I was a jerk for not "understanding," in spite of the other 99 times out of 100 where I was "understanding" to the point of being indulgent of her tantrums and terrible treatment. No real ownership of how awful her attitudes are. None.
Flip things around... . if you were under stress and were a complete

#$ to people you claimed to love, how would you respond? Would you blame and attack them for not being understanding? No, of course not. You would apologize and say something like, "I need to find a way to manage my stress better. I think it is becaues of all my stress related to school, but I'm sorry because it is STILL not okay to treat you that way. Please forgive me."
That is typical BPD. Everything in the relationship is expected to revolve around them and their needs and how you allegedly wounded their needs and didn't tend to them enough. Blame, projection, lying, gaslighting, etc. And they will use it to justify their rages, their lying, their cheating... . you name it. It's "your fault."
She's going to keep doing it to you... . forever... . and it will probably get worse. Sorry.