She kept asking me to be friends. I was reluctant to do so.
I would hope more than reluctant...
But I tried hanging out with her as friends.
Bad move. Bad bad bad move.
I felt like she was being flirty and giving me hope at times. I felt like because her bf works a lot she wanted attention.
Yep. She knows you want her, and you give her the attention. She's an attention-gal, and you don't want to ruin ANY chance in ever getting her, so hanging with her is your only choice, you feel.
The friendship looked like things were going okay, but then she started cancelling plans more within the last 3-4 weeks.
Yeah, because maybe she felt bad about it... and spending more time with her bf or opting for other more normal things to do... or just got too obvious & weird under the radar (which it ends up being).
Am I being a needy friend?
I wouldn't call it that -- you're not REALLY just a friend emotionally. You're being a guy who's chasing a girl who isn't into you.
Do I have the right to feel bitter from this experience?
No. You should feel bitter about things you DESERVED to have but were unrightfully taken away from you.... but that didn't happen You didn't deserve to have her... nobody 'deserves' to have anyone, let alone anything unrightfully taken away from you. You never had her.
She didn't screw you over... Although I would say her choices on being flirty and all that were not good, I wouldn't consider it a crime. But her actions were just as silly as wanting to hang out with a gal who isn't into you and dating someone, while trying to play CSI to look for clues on potential chances you so desperately want.