First and foremost great job owning your emotions and working to create positive change in yourself! You said that you know your responses are out of line with reality, but I worry that you may be selling yourself a bit short. Is your partner actually cancelling plans with you in favor of plans with someone else? That would frustrate me, depending on the circumstance, and would definitely be a problem if it became a pattern. Is your partner supportive of your need for validation and reinforcement?
Assuming that you are right, and this really is an overblown reaction to an emotional trigger from your past, I can recommend two things. First, it will help a ton if you deal with the source of the problem. Have you tried to resolve your feelings of abandonment from childhood? I worked through 'Toxic Parents' with my partner, and we both thought it was an amazing book that helped us heal. Another thing you could consider is cognitive behavioral therapy. You would need to see a psychologist, but they can teach you how to train the patterns in your mind to try to break the link between plans changing today and pain from childhood.
While you try to fix things in yourself, see if there is anything you can do for validation. Perhaps your partner could write some affirmations of their love for you, and you could read them over to yourself when you feel insecure? Perhaps just having your partner remind you that you are loved, and rescheduling an event rather than just cancelling it or leaving it up in the air would help.
Hope you find solutions,