The situation in the scene, Act I, scene i, is pivotal in understanding what Raina says here to her mother. Raina and her mother Catherine are just exclaiming over the first news of battle at Slivnitza. It was a victory for their side, and the victory was led by Sergius, Raina's betrothed (future husband). In the midst of their exaltation over a victory and over Sergius's role in the victory, Raina stops and in agitation reflects on her ideas in relation to Sergius and the war: "(She rises and walks about excitedly.)." Raina then remarks that as a result of the victory, she now knows her ideas were "real": "It proves that all our ideas were real after all." It is significant that she says "after all" because the words demonstrate quite clearly that she had doubt as to the soundness of her "ideas." When her mother protests, Raina goes on to describe what ideas she means and why she doubted their realness.
CATHERINE (indignantly). Our ideas real! What do you mean?
It is in Raina's explanation of her ideas and her doubt about their real qualities that she speaks the sentence you ask about. She has told her mother that she entertained a question in her mind as to whether in real life Segius could be as valiant and brave as their idealistic patriotism and love expected him to be: could the forces of real life, real war, real blood shed, real enemy lines to charge through on horseback or on foot allow for the patriotic heroism that "girls" at home would believe in to be the standard of behavior. She suggests that she has seen too many Romantic period plays and read to many Romantic period poems by "Byron and Pushkin" to be able to see and to think of real life in a realistic way. She implies that despite the inappropriateness of the thoughts, treasonable questions of Sergius's suffering "disillusion or humiliation or failure" played in her mind:
RAINA. ... I sometimes used to doubt whether they were anything but dreams. When I buckled on Sergius's sword he looked so noble: it was treason to think of disillusion or humiliation or failure. And yet—and yet—
Now that the context is firmly established, we'll direct our attention to the specific sentence you ask about: "Real life is so seldom like that—indeed never ...." Raina has said the she feared her ideas of "what Sergius would do—our patriotism—our heroic ideals" were "but dreams." She has said that she feared that her ideas were wrongly influenced by the external factors of the idealism of the Romantic period poets and their flights of idealist fancy. She is musing upon the descriptions and the depictions of life as presented in these Romantic works and suggests that her conclusion was that real circumstances and events don't match the Romantic poets' and playwrights' ideals of what real life holds.
In this context, Raina states her conclusion about the false picture presented of real life by Romanticists to her mother with the worlds, "Real life is so seldom like that—indeed never," with "that" being the picture she questions--that which is depicted by the plays and by Byron and by Pushkin and by other Romanticist--since she thinks it represents what real life is not like. In other words, Raina has concluded that real life isn't lovely and pastoral and loving and heroic but that real life is disappointing and humiliating and prone to failure and disillusionment. The rest of the play shows Shaw's ideas of whether her doubts or her revised ideas were correct and true: Shaw demonstrates whether life does accord with the Romantics' notions or whether it does not, whether it is in fact full of disillusionment, humiliation and failure.