I'm also a man in a long-term marriage that was hetero-passing and have relatively recently come to terms with being bisexual. Please don't be discouraged by the misconceptions people have about bisexuality. Looking at what the world sometimes communicates to outwardly bisexual people: you don't need to prove anything to anyone, you don't need to "be sure", you don't need to demonstrate your sexuality for it to be real, and you don't need to prove your faithfulness to your marriage.
To answer your question though: you might need to ask yourself what being bisexual means to you in practice and also what your wife's support and understanding is framed by. There is an assumption that there is an easy answer to how people will react to the news of something that was fundamentally part of their life-partner but sight unseen. Unfortunately there is no sure bet. If you are really worried, there are ways you can prime the conversation before disclosure to your partner though. For instance, you could mention a coworker or an old school acquaintance revealing their bisexuality to you and ask her what that might mean about them to test her perspective and reaction. Ease into the conversation by contextualizing and you will likely find her answer without revealing your sexuality. Honestly, some people will never understand and will not give the space to understand, but those that will become all the more important. Honest communication is important, but you should also be prepared for the consequences of that honesty.
I wish I had a better answer for you, but I'm hoping it goes well for you still.