I'm still new to this, but your situation sounds a lot like the one i was in a few years ago, so i hope you won't mind me chiming in.
I was monogamous for 6 years in two relationships, but always felt constrained by it, like i wanted more. I managed to be loyal, but it was a constant effort, until one day i just had to admit monogamy wasn't for me.
I'm poly, my husband is mono. Though we haven't added anyone else to our romantic situation yet, we have dealt with sexual openness. It's required a lot of work, and no doubt will require a lot of effort in future. But that's relationships, mono or poly.
The same is true for the fears you have. I know it's scary: I was terrified sitting my then fiance down to have 'the talk'. What if he didn't accept it? What if it ruined everything? What if i couldn't find anyone else: was i risking my relationship with him for nothing? But I'm a big believer that, regardless of the possible outcomes, honesty is always the best policy in relationships. I didn't talk to my ex, and wound up resenting and pulling away from him until he had an affair and broke up with me.
I would, however, wait until you've spoken to your therapist before having the talk. And be sure, when you do sit down together, that you're clear that this is not a reflection on him or your feelings for him.
At the end of the day, only you can decide what you want and need out of life. And your bf will decide what he wants and needs. And the pieces will fall where they may.
I truly hope everything turns out ok, and you can work through it together.