Whoa! Respectfully I say this.... you went from being with the "love of your life" to being single, to getting another girlfriend and becoming engaged to her? Wow!!! Take your time, man.
I'm with the above posters who said that your ex is "wishy-washy" at best. Your fiance should come first.... she is whom you intend on marrying. She also deserves to know the truth about the whole situation. (Take it from a snake in the grass, the truth is absolutely necessary-all ways!!!!)
Oh goodness---- rebound sure is rampant. When we leave a relationship by our doing or someone else's -- it is always best to not date for a while. The reason is that we are not emotionally available for a new relationship.
This is where you are at now. You can not be engaged to someone while still wondering if there is something there with an ex or longing to be with the ex. Whether the new person is a better choice or not, this doesn't matter in my opinion. You aren't emotionally able to fully be with the new person due to not having moved on from the past relationship.
My best advice to you is to be honest with your fiance. You can tell her how much you feel you love her---------- but I will say absolutely that between breaking up 4 months ago and moving straight to a new relationship with an engagement is fast by anyone's standards (another tell tale sign of rebound effect)------ if you didn't even have feelings for your ex, I'd still caution you to slow it down a bit. You can tell your fiance that you are confused and need to really get your head on better before you move forward. Then put it on hold. I'd do that for someone if I was talking about marrying them because even though it would hurt--- I wouldn't want them to be with me while longing for their old lover.
I'd actually love to see you take a break completely----- to not be with any woman and just figure out what you want and who you are without a partner (which gives us much better eyes to see what kind of partner would be best for us). Since I doubt you'll do that---- you need to be honest with all parties involved.
good luck
Well....hmmm......so, in essence, you are willing to sacrifice this WONDERFUL relationship with your fiancee for this "wishy-washy" ex gf who dumped you because she wanted her ex back? You want to take that gamble again? In my opinion, I wouldn't. The ex gf has PROVEN to be fickle-minded. Plus, this just might be some pattern her and her ex bf do. I dated a guy one time that stated he had been "back and forth" with a gf over a 14 year span. Of course, that was the first and last date I went on with him.
You are doing or contemplating doing EXACTLY what your ex gf did to you to your fiancee.
"Love of your life" and being "In love" with someone DOESN'T always mean that he/she is the BEST person to be with especially taking in consideration your past history.
If you are feeling confused, I definitely wouldn't keep this a "secret" from your fiancee. It wouldn't be fair to "string" her along with the pretense she is the only one you want.
Perhaps, you should be by yourself to figure this out rationally and methodically.
The fiancee sounds more reliable and better suited for the "long-haul" then this ex gf.