She rejected you. Good. Now you get to find yourself.
There's so much I wish I could Vulcan send you from my brain to yours. So many times I've thought 'this is the one' and invested myself to the point of desperation. Then years later I have minimal attraction for the same chick I was once head over heels for.
Here's the harsh part. You can't change somebody's perception of you overnight or even a short period of time. It takes seperation and reintroduction to start with a clean slate.
Don't be the readily available lap dog that is always there for her. You're her safety net and nobody aims for the safety net unless all other options have failed and they fall to your level.
Seperate yourself. Stop messaging her. Become interested in other people. It's counterintuitive but truthfully the best thing for you. Start talking to other girls, even if you're not attracted to them. That attraction will fade. You'll be introduced to other amazing women.
If your happiness depends on the acceptance of another human being, you're in too deep. Find yourself. Recalibrate yourself. Find happiness from within, not externally from others. The amazing thing about being young is the possibility to build a life worth sharing with another human being.
My advice is simple. Tell her you're too busy with X or Y to help her with her ex troubles. Don't demean yourself to that shit. That is your first step to cutting yourself free.
Second, meet new people. The sour pain will still be there for a while, sometimes a long while. It'll eventually fade.
Thirdly, love yourself. This isn't a 2nd grade 'love yourself' speech. This is a join a gym, take a walk, watch a sunrise, listen to music with your friends by a campfire, finding a hobby and fucking owning it, 'love youself' speech.
Lift your head up. Dust your clothes off. Lick your wounds. Improve yourself. Not for her or any other person. For you. You're a little stronger now. You're a little wiser now.
Live a life that you love independent from who you share it with. It's that confidence in yourself that will get women to tell other guys that they need their help to get YOU back.
You got this, no doubt in my mind.