Hmmm ... If you're having trouble convincing him, you may not have a lot of options as his skepticism is probably based in his emotions, and intellectual arguments don't always change emotions. To make matters worse, you're trying to convince him of something about your emotions (namely, that your feelings for him haven't changed). How do you prove that you feel a certain way, especially if you're not 100% sure of it yourself ("Can I *really*
love more than one person, or is this just my mind playing tricks on me")?
About the best we can do, when trying to determine whether someone loves us, is to observe how that someone treats us. If they treat us in a loving way, we usually conclude that they love us. So a lot of what your boyfriend may need is to be convinced over time by how you treat him. If you continue to treat him like you love him (and like you're still in love with him), he'll probably start to feel like, "Hey, this is okay, she really does still love me." You yourself may even be able to see, through your own behavior, that you really do love more than one person. It stands to reason that if you can treat two people like you love them (and like you're in love with them), then you probably do love both of them. It's how we "measure emotions," is by looking at behavior.
This doesn't necessarily mean that you have to engage in a threesome to prove you're in love with two people, but it does mean that when you're with either one of those two people individually, you're able to treat that individual in a loving way (and it doesn't have to be forced).
Convincing your boyfriend about your feelings is mostly going to be limited to demonstrating your feelings for him by how you treat him. As for yourself, only you have the ability to look inside and see how you feel. Other people can't be mind-readers. So how do you feel? and if you do identify a feeling inside, do you immediately start doubting/questioning it? Perhaps you're second-guessing yourself. Why not trust that the feelings you can feel inside are real?
Maybe you're afraid that poly people are just making up excuses for cheating? that we want to sleep with someone else, so we (subconsciously) try to convince ourselves that we still love our original partner? That would be the mind playing tricks on us. But remember there are many kinds of tricks the mind can play, including social conditioning. Most of us are "trained" throughout childhood (and adulthood) to view monogamy as "the only way things can really work." That's a trick the mind plays. Your mind could be playing tricks on you by pretending it's playing a trick when it's really not! Pretty confusing, but just as "real of a trick" as any other.
The bottom line is, you could be second-guessing yourself all day, but the best things to do are probably:
- Trust your *first* thought about things; your first thought is usually the most accurate about you.
- Let your behavior serve as your evidence about how you feel.
Of course you can (and should) also reassure your boyfriend that you still love him (just as much), and that that hasn't changed. But you can't "prove" how you feel ... He'll just have to take your word on it. Over time, he'll be able to see through your actions that what you're saying is true.
Plenty of people (on this site and others) can testify that love can be a plural thing, and I can add my testimony to that. For what it's worth, polyamory (loving many or being in love with many -- honestly and ethically) is a real phenomenon; it really happens. But my saying so doesn't prove it to you (or to your boyfriend); you have to find out by your own experience.