OP, remember this one simple phrase “attraction isn’t a choice.” That has guided me into many successful interactions and has guided me out when I overpursued or acted too interested.
You can hit the gym all you want in the next 2-3 months and dress better to “convince” her that you’re better than her ex or orbiters... truth is... you should pursue change within yourself because you sincerely want to be a better man. You should want to change for the better because you value and respect yourself.
Playing “ghosting” games with a woman you’re interested is not manly behavior coming from that frame of neediness looking to manipulate her into feeling attracted to you when she hasn’t.
Did she do anything reciprocal to warrant you putting your arm around her that one time? It sounds like she used you at work to get something for her. Women are more attracted to a man that she has to earn the trust and affection from.
Like others have said, go out, talk to other women and begin reading the DJ bible. You’ll become more outcome independent.