Males and females, how do I get this guys attention again (Long read) ?

Ok I (23 year old female) liked this guy at university but I was too shy and I think my shyness masked my outward attraction towards him. He never noticed me at all but I think it was because I was too shy to flirt with him so I would pretend he didn‘t exist. One day I decided to flirt with him by maintaining eye contact and this was the first time he noticed me as he also held eye contact with me smiling a bit but I didn‘t smile back (shyness). Every time we would walk by each other I noticed he would try to flirt with me via eye contact but I was too shy and would walk past him as if he didn’t exist or look in the other direction which probably confused him. He never approached me, only trying to get my attention but I was always nervous to make eye contact with him so kept looking away. There were times when he would see me with a group of guy friends I could tell he was a bit jealous but he still tried flirting with me and I stupidly kept ignoring his flirt signals (again shyness).His constant flirting lasted for a long time but he never once approached me, just eye contact flirting and constantly trying to get my attention. I could tell he was looking for opportunities to approach me but I think I gave him the impression I weren’t interested by ignoring him but he still kept on flirting and looking for opportunities to approach me and I noticed that he slowly started to back off. I left the college for a month and came back and I‘ve noticed that his completely backed off (stopped flirting) and doesn’t acknowledge my existence anymore when I walk past him/ in his presence and now another girl is trying to snag him but she‘s being more aggressive than me. I’ve began trying to flirt with him but he no longer flirts with me anymore. How can I get him to approach me/ make a move?