Dear Mara Moutain,

Remembering your dreams is not the same thing as lucid dreaming.
Sometimes when a person has been a huge part of your life and now he is nolonger there, he is there in your memories and therefore your dreams.

Also there are many times we think we have resolved matters of the past but truth is the past is still part of who we are today, an we need to embrace all the good with the bad and come to a place where we recognize the lessons learned. That is what your dreams are telling you, he keeps showing up means you have more to learn about that relationship before you can truely move forward. If there are unresolved painful emotions about this person your dreams might be warning you what behaviors of him hurt you and investigate before entering new relationships.

As for lucid dreaming, it takes alot of mindfulness to know when you are dreaming.
First you must be able to master thinking, thoughts, feelings, and action in life.

Then when you are aware that you are dreaming, it is important not to react, just watch. Do not respond emotionally or interact just watch.
Later let the dream playout, watch how you are being in this dream as well as others.

There are always clues to recognize when you are dreaming, yours is when you see your ex, you can calmly say I am dreaming. I wont be pulled one way or the other.
This takes time and while you are awake have an openview, be mindedful, and non reactive to your emotions.
These are some of the tools needed to be a lucid dreamer.
A lucid dreamer is one who is in control of their dreams.
Best wishes