Sorry to hear you're in a such a bad place at the moment. Killing yourself would 100% not be a 'right decision' though. Doing that would only remove all of the potential of moving forward from here and finding a new happiness.
I believe that you truly loved this girl but I also believe that there is more than one person that we can love. Yes, you need to sort yourself out first but after that there's no reason why you won't find someone else who you have that connection with and who makes you happy.
I also think that you do know that the way you behaved towards her after the breakup wasn't acceptable, but that it came from you being in a bad place rather than any intention of causing her distress so perhaps don't judge yourself too harshly for that.
Please reach out to someone in your family or to a professional about how bad you are feeling. Give them a chance to try to help you through this.