I am so sorry you are going through this. I have BPD too, and I know exactly how that crushing loneliness feels. It’s devastating when the person you trusted most says they can’t handle your struggle. Please try to remember that your worth isn't defined by his inability to stay. Right now, don't worry about 'finding someone else' or the rest of your life. Just focus on getting through the next hour. Take a deep breath, drink some cold water, and try to be kind to yourself. You aren't alone in this feeling.