How can I just let him go?

the problem is you. You are attracted to this man because you have a self destructive streak.

blah blah blah

focus on school, get a hobby, maybe a pet - do something to stay busy

First of all, what you are experiencing is a combination of infatuation and a possible soul tie. The infatuation isn't so bad but, the soul tie has to be broken, so you can move on. Simply ask God to show you if there are any unhealthy soul ties in your life. He will give you an image that fits you, like water hoses, or pipes, or something attached to you. Using your imagination, sever them and attach them to Christ, so he becomes the source of whatever each of you have been getting from the other.

Real love is no more than 20% physical (appearance, affection, sex, etc.). It is around 25% emotional (feelings, risk, intimacy, investment, etc.). It is around 25% mental (decisions, thought discipline, commitment, etc.). The largest aspect of love, making up more than 30%, is spiritual. Spiritual intimacy means allowing someone else to reach that level of depth, in you, where your identity is no longer protecting the real you. It requires trust at levels that leave you absolutely vulnerable to another person, or God Himself, with no protective mechanisms or barriers in the way. It requires intimacy between your spirit and the spirit, or Spirit, you seek intimacy with. It requires the investment of everything you have decided, developed, and learned to utilize to protect, and identify, yourself, in a relationship that will redefine you into the person you were meant to be.

Love does not force itself on anyone. It pursues you until you recognize its desire is not to hurt, but to heal, affirm and develop the real you. It allows pain at times in order to peel away the layers of identity that don't belong, which keep us from becoming who, and what, we were made to be. It is patient, kind, long suffering and quiet. It does not get upset when you scream at it. It does not dismiss your pain, but seeks to lead you through the pain to the freedom on the other side. It does not ask you to cope, or protect, but to trust and allow it to protect the real you. It never forces its opinion on anyone, speaking in a soft, affirming voice, differentiating between you and your behaviors, affirming you, even while confronting your behavior.

The best way to learn about love is to read the ultimate love letter. The Bible is not a law, a historical fiction, or a hammer to beat people with. The Bible is God's love letter to us, in which He introduces Himself, through various means, including history, poems, letters and other types of accounts that prove He desires intimacy with us at levels which can never be reached with anyone else, without reaching them with Him first. Love is what allowed God to become the object of His own love, so He could die and become the wrath we thought He held toward us. Jesus doesn't ask us to die, or kill, for Him. He doesn't ask us to become less than what we were made to be. He asks us to trust Him, and Him alone, so we can be free to live the life He made us for.

Try to meet him first.