1st of all, ask her. Be vulnerable and let her know that's the case. Insist she be honest and that it won't hurt your feelings if that's the case (even if it would). If you trust her and know she loves you, I'm guessing you'll tell if she's being honest. If not sure and/or hearing an answer you don't believe, just push for the truth.. to a point. At some point, you have to choose to believe her.
If she's honest and tells you it's just not the same with you being so much smaller, it's no reason to be insecure... unless that's your turn-on, of course. lol. There's nothing wrong with accepting your role and admitting you're there as support, but will never be the main attraction.
Even with that being the case, there are many ways to please her. For one, you could find a strap-on dildo that's much longer than you and as thick as she can handle while strapped to your cock so you can essentially give her double vag - good for you and for her. Obviously making sure your oral skills are exceptional is another thing you can do to help her. And last, but not least, you can keep finding her other, bigger men. All in all, she'd be crazy not to love you even more for being so giving and taking care of her so much.
That's just a couple of ideas based on very little knowledge of your reality, of course. Only apply it as you feel would be beneficial. Otherwise, feel free to ignore.