Where did I go wrong with this girl? Is there anyway to get her back?

I really messed up with this girl. Is there any way to fix my mistakes and get her attention again?

1h
Im a senior in hs. This girl is a sophmore. She used to like me a lot last year but i never caught on and I was interested in someone else so i never realized.

Over the summer i realized i missed her and i actually have feelings for her, so when school started again i started talking to her a lot more.

We were talking a lot and everyrhing was going great. I had it in the bag for a couple weeks, but I waited too long and she started to lose interest.

I panicked when i saw this so i texted her and told her how much i like her and asked her out. She said she likes me and im awesome but now isn't a good time. I said i understood and all but maybe we could go out in a couple weeks, to which she said yes.

So i waited a month or two and kept talking to her. She starred to go hot and cold on me. One week she really liked me, the other week she acted less interested.

I asked her to he movies recently.
She told me im awesome but she's busy and ahe has feelings for somebody else.

I know the guy she likes. I don't know how things are between them. They talk in school and he's friends with her older sister (hes a senior too).
After she told me she likes someone else I said i wish it could have worked out & i really regret not talking to her more last year.

She didn't respond to that, then she texted me Happy New Year with little hearts and all at 2am the other night.

Weve had school again for two days and i saw her talking to him once or twice.
I haven't talked to her but I noticed her looking at me a couple times, watching my stories, liking my posts

I realize i screwed up, and I don't know how to proceed. Im moving on to two or three other girls (one is actually a really good friends or hers so maybe she could help me there) but I still hold out some hope that things with this other guy dont work out.

The friend is a beautiful girl i see around school a lot. I actually liked her last year and one day i asked her out without ever talking to her before. She said she had a boyfriend but after that she would stare and look at me for the rest of the year.
Looking back on it, i feel like i probably caught her way off guard and maybe weirded her out. I was thinking of just approaching her one day, telling her shes beautiful and joking about how i came off strange last year. Hopefully get her number or snapchat.

However, i dont want the girl who just rejected me to think im trying to just get back at her or make her jealous. So i think i should probably ask her first to help set us up ad a mutual friend?

Eithr way, to get back to the point, while i go after other girls, whats the best plan for this one girl. Do i stay friends, cut contact, start talking to her friend i like?
Where did i go wrong with her and whats the best move now?