Please I need advice, I have a possessive friend

I'm a straight woman (21) and I have a friend that I've known since high school. They have always been clingy and negative in general but I've always thought it was just their personality. We used to have conversations about how difficult they can be. If I don't answer my phone they keep on calling and calling and when I do they get upset asking where I was. Every guy I have dated has never been good enough in their eyes. I've told them how negative and frustrating they can be but they continue to act the same. I moved away for college and  during freshman year my friend started to date this girl who they introduced me too. They always want to Skype as a group and when ww do they spend hours complaining. About 3 weeks ago we were skyping as a group and my friend came out as trans and asked me to call them by male prounouns and a male name. With the urging of their girlfriend my friend confessed that they had a crush on me in high school and thought they could be the perfect guy for me. I firmly told them that it would never happen but they will always have my support. I won't lie it made me uncomfortable because all these years I thought they were just difficult not that they liked me. Yesterday was their birthday and I messaged them with a little message and birthday video. Later on at night I was in the library working on an assignment. I didn't finish it till 1 am and I saw that they had texted me asking me if I was free. I responded then asking how their day was. The friend and her gf then messaged me saying that they had been trying to contact me through skype and text all night and that they wanted to skype with me to celebrate their bday. I was so frustrated. I just apologized and went to bed. I feel this friend treats me like I'm their girlfriend and I don't know how much more clear I can make it that I'm not obligated to tell them where I am and who I'm with?