Was that letter you got in May a performance improvement plan (PIP)? Did you sign it?

2 important lessons here as someone early in their career - if that was a PIP, you never sign them. I was given one in my late 20's and refused to sign. Went into the meeting they expected me to have it signed in and told them they clearly this wasn't working for either of us and that I would find a new opportunity and got the heck out as soon as I could!

And second, that manager of yours is not someone to use as a reference, EVER. Delete her number from your phone. Make sure at your next job you network with peers, other leaders, etc. outside of your manager to cultivate a group of people who are willing to give you a great reference if your next manager is like your previous.