It's been over a year since we broke up, but sometimes I feel like it's still possible to get back together. How can I open the door if that's the case?
Your feeling, does it come from your intuition, or it's your hope?
You may be also picking up your ex partner's feelings, if you did not severe links to her/his chakras.
How to open a door is too personal question, generally, it may be easier than you think if your feelings are true.
Ask yourself why you feel like you could get back together and what you hope to accomplish by opening up a door. After some careful consideration, you may find that you can rebuild what you once had if you both have figured yourselves out.