In Act II, Sergius exclaims to Raina that she is his "lady and his saint." When she begins to respond, (saying "My lord and my-") he interrupts her. Why?
This is our first hint that Sergius isn't just disillusioned about warfare. He also has misgivings about what Raina expects of him. He is finding those expectations onerous, a sentiment he'll explore later: He doesn't like being admired for something he's not, or held to standards that he can't meet.
But for now, during this interchange, he isn't ready to confess why he falls short. He isn't ready to disavow his image as the heroic lover, or consider the possibility of terminating their engagement. He just wants Raina to tone it down, so that he doesn't feel so sharply the difference between the image and the reality.
Notice, for instance, the way he frames his request:
SERGIUS. Sh--sh! Let me be the worshipper, dear. You little know how unworthy even the best man is of a girl's pure passion!
He doesn't speak of his personal inability to live up to Raina's expectations. He speaks in generalities of what the best men are like, leaving Raina free to infer that Sergius is one of these men. And because he doesn't actually claim to be "the best man," his comment can be interpreted as admirably humble and chivalrous. In effect, he is telling Raina not to praise him because women are superior. Sergius may be incompetent in many respects, but he knows how to choose his words in ways that portray him in a flattering light.
In addition, the stage directions that follow indicate that Sergius has been speaking to Raina with the "loftiest expression," reinforcing the impression that Sergius is happy to be perceived as a great lover. So while his interruption betrays his internal conflict, that conflict hasn't yet come to climax.