You've already been given some fabulous advice already, which I won't repeat. What struck me the most with your post was this part:
Quote Originally Posted by aly View Post
It was so discouraging. I got super-negative. All I could think about was how hard I've been working and how I'm still so slow and I'm passed constantly, and although everyone is nice and supportive, it gets old being the slowest and so on. My mind just kept going along these lines and I got very emotional and negative.
Do you generally have issues with negative self-talk? I had a riding partner who really struggled with self-esteem and negativity, and this reminded me a lot of her. What I find is that being in positions of extreme distress brings out all of my usual "demons". Having an incredibly challenging ride can get you to that point of meltdown alarmingly quickly. They're there with me every day of my life, but they really rear their ugly little heads when I'm on the edge. Anyways, this might be something you want to explore in therapy/counselling. It's helped me out a lot.