Ok, I've had this dream and variations of it for a while. Even before I met my fiance, I'd have these.
My relationship with him went like this:
We met and had sex on the first date, he told me he loved me then and I let him, and said it back when I knew it wasn't true. It was my first relationship and I wanted SOMEONE, not really him though. Within two weeks I broke up with him, and it was horrible, I didn't want to hurt him.
He was suicidal, and I had to go check on him to make sure he didn't hurt himself. I didn't know he was this way until we broke up. I found out later that after me he had a fiance and when she broke up with him, he cut his wrists and almost died.
He begged me to try it again the next year, and I stupidly let it happen. Same ending. I had a hard time breaking up with anyone after that, I was always more concerned with how they would take it, but I always did it the second I knew it wouldn't look out so I wouldn't lead them on. Or not let the relationship start in the beginning.
So my dreams are always something like I'm with him again, and I don't want to break up with him for fear of hurting him, even though I want out so bad. Now that I'm engaged, he is my fiance. My ring, our house, planning the wedding etc, and instead of it being the face and personality of my fiance, it's my ex. I can't get over how much I will hurt him to give him the ring back, because we picked out the ring together, I know how much it cost etc. And his family, I love them and I couldn't handle everything changing. Even having to move out of our house.

I wake up in a sweat and almost in tears until I see my fiance and am relived because it wasn't real and I am so happy with my fiance. But I hate explaining to him that I just had a dream that I was engaged to my ex. I don't know what my issue is, but I'm getting sick of this. I'd like to know why I keep replacing him with my fiance, and before, my boyfriends. These dreams are really realistic, and it takes me a minute to come back to reality.

Sorry for the wall of text

Dreams are symbolic expressions of our feelings about our waking life at the time we had the dream.

You were clearly traumatized by your experience with your first boyfriend, which has strongly imprinted itself on your memory. That would explain how your ex becomes the symbol for other boyfriends, including your fiance. An ex often symbolizes qualities that you associate with a current romantic interest. Here's a link to the definition of an ex on this website: dreamdoctor.com/index.php?option=com_dre...&task=details&id=726 There is also a great video on the homepage of this website about what ex-lovers mean in dreams.

As for why this would always be the symbol for your other boyfriends and even your current fiance is a good question to ask yourself. The reason might relate to feelings that were triggered deep down by an event, a comment, or even a look from your current boyfriend... one that recalled your ex or possibly the demise of that relationship. That is not to say that your ex's appearance in your dream spells doom for your current relationship, but it does reveal how sensitive and concerned you are that your relationship might go that direction. Try to ID what occurred in waking life just prior to the dream that triggered those feelings or the association with your ex. The dream should provide clues to help you identify what it was.

It is also worth considering whether or not the regular appearance of your ex in your romantic relationship dreams is a symbol of your concerns about long term relationships in general. Your ex may symbolize your dread of bad relationship outcomes... the inevitability of a painful breakup and/or your sense of responsibility for the feelings of the other if and/or when it doesn't work out. Something in waking life may have stirred deep seeded feelings of mistrust about your romantic relationship based on your past experience. Who symbolizes that better than your ex?

Awareness of what the trigger is and how it is stirring this negative association in your subconscious may make these dreams go away... or it may require talking about it out loud with a professional, who can help you understand and process your feelings so that your ex will disappear and the one you truly love will star in your dreams.

Hope that helps!