Been wanting to fuck this girl at my Uni for 4yrs because of her absurdly fat ass, cold approached her last year in summer but I quickly retreated because during this conversation I realised she was a friend of a plate at the time

Since then its been constant up and down. I hard next her, then we run into each other couple months later (usually just short encounters), she's flirty and I think "she's not that bad, next time I'll make a move", then too much time goes by and I put my priorities elsewhere or she sees me with another chick and decides to start ignoring me so I hard next her. Until a few months later we happen to get into an elevator together or something and all of a sudden she's super flirty again.

Her friends have been obsessed with me this entire time, constantly giving me looks etc. and asking my (now ex-)situationship about me, if I'm single etc.

Couple weeks ago I ran into her again, she didn't seem that interested this time so I gave her a compliment to make my intentions clear and then left. After that I saw her smiling at me a couple times but didn't go up to her because I dont like handing out free attention and it didnt seem like she wants a date.

Now last week I saw her again, she was very flirty, complimented my hair. I wanted to ask her out there but got caught off guard by my train arriving so I had to leave. Then she "caught" me turn around and check out her body (I did this on purpose and wanted her to see it) and responded with a big smile on her face.

I cold texted her a couple days later & asked to go for coffee. She took 5 days to reply, said she's busy this week, has a bunch of stuff going on and that she goes on vacation with her boyfriend next week, so she doesnt have time for a coffee. Her response seemed weird to me, she pretended I was just asking platonically and I dont think she mentioned the bf before

Now I'm kinda not sure if I should keep escalating when I see her or if I'm just an orbiter and giving away my attention. Am I being delusional thinking this chick wants to fuck me?

Attempt to answer own question: I probably have a case of oneitis and if another chick gave me these signals I wouldn't even waste time thinking about her.