Hi there! I am sorry that you are struggling with your daughter's situation, and that she is feeling so bitter. It is a tough place to be for her, and probably even tougher for you to watch and to feel so powerless to be able to fix her situation. As a parent, all we ever want is to see our children find happiness and feel confident and successful--- and we will stop at nothing to help them in anyway that we can.
The answer to this situation, from my vantage point, is that she will need to stop facing life as a helpless victim with zero power and to begin working on her own self-confidence in order to attract more positive energy and people into her life. Negativity, bitterness, a sense of entitlement and harboring a grudge toward life is going to attract the very same. Her self-talk may need some adjustment; telling herself that Mr. Right is right around the corner, but that until she loves herself and becomes fully confident in who she is as a single person, she will continue to turn away anyone with positive energy who may come her way. Also, she must take action to connect with like-minded people, meaning online dating sites, speed-dating, joining groups and clubs or service organizations... taking action is the antidote to feeling powerless.
As for you as her mom, you have every right to lay down a boundary for how you will allow her to treat you. If you feel that she is being abusive and that her anger is misdirected, you can let her know that you cannot tolerate this any longer. You can be willing to offer any help she needs, but you are allowed to have your limits of tolerating unacceptable behavior. If you feel she is on the border of depression, be honest with her and offer your help to find her a therapist. The toughest part is just accepting the fact that there is little that you can do to solve this for her, but you can build her confidence with compliments and spending quality time together. Although finding love is tough sometimes, as soon as she becomes comfortable and finds happiness within herself... by herself... that happiness will shine through and will have an effect on her relationships.
If you have further questions or information to share, please just reply and I will answer as soon as I can. Otherwise, I hope you found my reply to be somewhat helpful to you! This too shall pass... with a slight change in thinking and perspective, she will eventually find what she is looking for! Hang in there!