The first step towards solving this fixation over someone is to truly get over the idea that your fantasy don't equate to reality.
When you have a crush, its completely normal to be blindsided and only be fixated over all the positive trait that your crush has. It is important to note that, this is completely dangerous as it can result in obsession and not a true reflection of what the other person is.
Your crush should be treated as a normal human being, no one is perfect and it is important to suppress those emotions and lusts that you have and try to befriend the person instead of staring and obsessing at that one person.
This fixation is completely normal. But you need to surpress it and treat the other person as a normal being.
You are an adult it is important to recognize what you want from a relationship.
Be gentle with yourself, dont hurt yourself and get fixated over a mistake that you did. Be normal.
Dont be irrational.