I'm in a super weird situation. I'm dating someone but the person I truly liked and he first chose someone else is now single and I just dream of getting back. What can I do?

Well, you need to review your relationship and see why your partner left you. If a person left you for someone else it does not appear to be trustworthy and most probably you will not have any future with that person. sometimes we are in relationships that just filed our needs and we do not see broad future in it but we do not want to accept it and move on. we get somehow dependent and can not free from them. I would give myself distance and focus on my own health and life for healing my soul and move on.