How to exert confidence/dominance over girls who treat me like shit?

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22 male average guy in university somewhat awkward and quite isolated, but with an iron will and refusal to give up.
I lack confidence, but still try to give everybody a chance. There is a girl living upstairs (My housemate) always always looked out for her, when she was sick, I was there nursing her. When she was depressed, I was there talking to her. Not interested in her sexually or romantically, but always tried being there for her.
She was a girl though and I always felt somewhat uncomfortable around her.
She always displays affection towards me through words, but ignores me completely when she doesn't need me. Had parties upstairs and never invited me.
She would go out with her friends knowing I had very few friends (New to this country as a student) still didn't care.
Comes to me when she needs me though.
I felt like shit naturally so I pushed her away and focused on my life, she starts writing me all this shit about how I am so sweet and that she misses and loves me. I forgave her and became just as nice with her.
One day I went upstairs to say hi to her, she was with her friend. We talked for hours, she reveals how she always wanted to do a drug with me, I was excited and happy. Even months ago she told me how she wanted to do these drugs with me. So naturally that night when she told me we would do it over the weekend, I was happy since I get to have a social life with her friends. Was looking forward for it. When night finally comes, I hear nothing from her. No message, no nothing. I find out she is out partying with her friends. She doesn't care if I am even alive.
I message her later and ask what she was up to during weekend, she replied feeling somewhat guilty and asked who I was with?
I lied and said, ''some friends'' asked her about the drug.
She simply doesn't reply, ignores me.
I knew she was moving out to another place, she told me, but then she leaves this place after the weekend without even telling me.
I feel like shit.
What do I do? :(