This can hardly be a unique experience, so for the benefit of anyone who finds this post later, I'll offer a couple of words.

If you've been a good and involved father, I think it was wrong of your daughter to do that. Sometimes children can consider a step-parent to be their real parents, like my half-sister, whose dad left her as a little girl. But we as a culture have a real misunderstanding of what a step-parent is, I think in part because, semantically, "parent" is in the word. But some terms don't actually mean what they sound like. For example, "second generation immigrant" (I am one myself). A useful term, to be sure, to describe my unique experience, but a second generation immigrant is not at all an immigrant, even though "immigrant" is in the term. I am native-born. I should not be thought of as an immigrant, and in this case, I don't think the step-father should have been thought of as a parent. The semantics are a reason to show your daughter some grace. Definitely do what others are saying and speak to your daughter about how you feel; she's an adult.

That said, a step-parent can be a sort of stand-in for a parent, and given your description of how you were as a parent, I definitely think that's the case in this situation, in both life and in the symbolism of him also walking her down the aisle. (By the way, weddings are so performative, like a stage play. Show your daughter some grace for this reason, too.) I suppose she said, let's recognize the understudy, too, by letting him play the part alongside. If anything, you were such a great father, she wanted you there twice, and he was the additional stand-in for you.

That may not be a good thing that you were such a great dad that you got to be there twice. It speaks to what others are saying, that you've sacrificed your happiness too greatly. I agree that you need to seek counseling, and I am pretty anti- counseling. After all, her mom loves your daughter as much as you do, right, and does that bother or threaten you? Why her step-dad loving her bothers you is definitely some work you need a counselor for.