Anyone ever heard of the Pygmalion Effect? In short: if you have a belief about somebody (ie: doubts about my significant other/they will leave me eventually), it will cause you to **subconsciously** behave in ways that will increase the probability of that belief to become a reality.

Another example (taken from a YouTube video) : Let’s sat you are a basketball coach, and you believe that Bob on the basketball team will be a great player because he reminds you of a famous basketball player vs Todd who you believe won’t be a great basketball player. You will be more likely to encourage Bob to practice more and other positive things. But the key point is your belief of him *may* cause you to do all these things subsconciously. As for Todd, because of your beliefs you are more likely to not be as supportive. When you are acting in ways towards people’s based on your beliefs towards them (which aren’t necessarily true) over an extended period of time, it’s no surprise that the belief has a higher chance of becoming a reality. However, most people when that belief does become a reality conclude it to their intuition being right rather than acknowledging and attributing it to how their behavior subconsciously changed the way they act towards another.

Where I’m trying to get at with this in regards to BPD: if you are doubting your partner and that he/she may do this or that and constantly have a paranoia that they don’t love your or they will leave you with no logical reasoning, it will influence your behavior towards them subconsciously and ultimately increase the chances of it becoming a reality. But most people with diagnosed with BPD, in such circumstance would attribute it  to their intuition being ‘right all along’.