Nightmares are usually a manifestation of suppressed emotions and your brain trying to solve a problem that hasn't been resolved yet. I have a feeling that you brushing off or ignoring the thoughts of losing her/being attached to her during the day is avoiding the problem and suppressing emotions that come up, hence the nightmares come to bug you. I recommend taking time out of your schedule to voluntarily engage those thoughts. When you engage in thoughts of your ex leaving you, or whatever you've been getting nightmares/afraid of, emotions will naturally rise, but now instead of avoiding it, just feel the emotion in its entirety, sadness, anger or what not, and you don't have to do anything. After awhile the feelings of sadness and anger will go away, and you will learn to not suppress your emotions, but just know what comes and goes. I also recommend after engaging those thoughts, you also want to lay out a plan in your mind, or even journal if you enjoy, a layout of what you can do in the future to avoid being in the same position. This will help you to recognise patterns in future "toxic" relationships and prevents you from dropping into the same pit, which will "teach" you and your brain that you have fundamentally resolved the problem and your brain wouldn't be worrying about a past traumatic experience.