It sounds really hard for you right now, on one hand you are able to see where the relationship wasn't good for you, but on the other hand you really cared about that person. Break ups are can be a blow to anyones self esteem but when you have a mental illness it can make it fee much worse. Did you feel like this after your previous relationship ended? Was there something that helped you feel less hopeless?
I wonder if you are getting any support right now? It might be worth increasing that support as seems like you're saying that these low feelings from the breakup aren't getting better. If you feel like you have no one to talk to about it you can call the beyondblue helpline on 1300 22 4636.
Also, I know that you've probably heard this enough, but I'll say it anyway (feel free to ignore if you're sick of it). People with mental illness can be in strong and healthy relationships! You sound like your willing to put yourself out there, despite having set backs and that's really brave.
If you feel comfortable, let me know how your going.