I read it, I hear you. To me it sounds like you have recognized that you have poly tendencies while trying to operate in a mono world.
Take a step back and breathe. Think about what would really make you happy. Think about if the decisions that you are making now take you further along that road to happiness.
When you found yourself having feeling for both L and B you broke it off with both of them. Did you give each of them the option of staying with you? Is that terribly different from the situation you find yourself in now?
From an outside perspective - being honest with the people that are important to you is vital. If someone is going to be in a "real" relationship with you they need to know the "real" you. Otherwise they are in a relationship with an imaginary fictional persona. Which isn't fair to them (or to you).
Just because you think you may be jealous if your partner is with another doesn't mean that you can't let them know that you can envision situations where you wouldn't be. What if they have been having the same thoughts but don't know how to express them? (PS. don't count on this, many never have considered poly as a possibility)
Again, step back. 5 years from now how do you want your life to look? Do you want to be having the conversation with your current GF that 5 years ago you had these feelings and you never voiced them and NOW (5 years later) you feel the need to explore your poly tendancies that you have kept bottled up and secret for all this time?
Just askin'. (You don't have to answer - just giving you some food for thought). I didn't share my growing attraction to Dude with MrS - I did it wrong (you can read about it in my blog). Luckily for me it worked out - it has worked out for others - it has failed miserably for many. Ultimately, this is YOUR life - how do you want to live it?
Random thoughts from...
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe