You have thoughts and feelings, they do not have you.

Source

When upset, people's unconscious thoughts have a tendency to "leak out" into their word choices. A person with a growth mindset is not concerned with outcomes when judging themselves. They are concerned with how they went about doing what they did. If a person gives something the best that they can give it, that's all they can do, and they're happy with that, while simultaneously realizing that "the best that they can" will keep getting better and better as skill, knowledge, and wisdom are acquired. This differs from how you seem to be approaching it because you have yet to release your judgment of yourself from the outcome. It is apparent to me from your original question that you correlate your own value with the outcomes you hope to get. People stop too soon when imagining scenarios, which causes them to forget that there is an after all of that bad stuff they imagine in which things get better again and they'll be fine.
Negative feelings and thought loops are where mindfulness can come in. Much of the time we get "trapped" in these things and so caught up in "being in them" that we don't see that they're only thoughts and feelings. As one becomes more mindful they start separating their idea of who they are from their thoughts and feelings. They start seeing thoughts and feelings as things that come and go just like everything else.

Thoughts and feelings can come and go and you can see them as objects that are not you. This separation is pretty key. The bodymind thinks and feels all sorts of things, but those things aren't who you are. They are appearances in your awareness and when you can see them as such then you can choose to assent to them or to let them pass away just as quickly as they came. To see them this way you need mindfulness, and mindfulness is just like any other skill: it gets better the more you practice.
I want to make sure my points don't seem to indicate that people with growth mindsets don't feel any disappointment or unhappiness, because that's not the case, but they are happy with their best and can deal with a little unhappiness. When coupled terracore with mindfulness, this unhappiness can be seen as just another appearance in awareness that's free to come and go without you assenting to it.