There is a darker interpretation of this song than I have read around here. This is not to detract from the beauty of the composition, but rather to add to it. It was written in January, 1964 by a married John Lennon, who admitted after the recording (2/64) that it contained autobiographical elements, and that the ballad style, and the weaving of coded personal elements was inspired by Dylan.
Musically, the Intro is a vaguely structured descent (Ebm, D, Db, Bbm; Ebm, D, Em, A), that goes in a circle and seems to waver around and try to hide the song’s key: D. It’s “beating around the bush” a little bit musically, as he sings the intro. He doesn’t want to reveal the song’s key yet for some reason because it's a confession. Exactly one year previously, he burst upon the scene and quite directly said (in the same key: D), that he all wanted to do was “hold your hand”. Things have progressed. Now there is more to it than “just holding hands”. “If I fell” takes on dual meanings: falling in love, and a fall from grace. If I Fell = If I eat the “apple” of temptation and cheat on my wife.
It's not too much of a stretch to say that the lyrics are the contemplation of converting an adulterous affair into an eventual replacement of his wife, Cynthia. Based on the linkage with "holding hands", this affair has been underway for a long time, and in the song John contemplates their next steps.It has become much more than just holding hands. He makes excuses for feelings which he knows are wrong by finding fault with how Cynthia is treating him, such as revealing that he feels she doesn't love him very much ("I must be sure...that you would love me more than her") and that he is mistreated by her in some way ("Don't hurt my pride like her, 'cause I couldn't stand the pain").
What John is saying to his new love is that he wants to leave his wife but only if she promises "to be true," the source of his dissatisfaction with Cynthia. The vulnerability in John’s pleading is ironic. Lennon has a very contractual idea of love with a passive/aggressive communication style. The same emotional oxymoron found another telling later in “Don’t Let Me Down” (“I’m really in love with you” in soft and pleasing tones, then screaming “DON’T LET ME DOWN!” like a threat.)
In the case of “If I Fell” we have Paul singing the upper half of a song which seems to have two melodies that intersect to form a single melody in the harmonization of the two parts. (See another example in The Everly Bros “Dream”) Although it doesn’t sound hard, anybody who has sung John’s part can tell you how incredibly difficult it is, and how much discipline it requires to “not go high” and follow Paul after the unison parts
The version we all know was sung live into one mic at Abbey Road and is on one track of a 4-track master. The technology exists now to separate the parts. Having heard both Paul and John’s AI “isolated” parts, they almost sound like different songs emotionally. Paul is singing the happy part of love, and John is singing the pain. The harmony of those emotional poles forms the melody that we hear. Together, they summarize John’s anguish with what he feels he must do, and despite his optimism in his new love interest, he is exposing his vulnerability and need for reassurance.
John has already "fallen" in his own mind: Julian was already 8 months old when he wrote the song. The relationship may be fantasy or John's imagination, but the "fall" is already there. Perhaps John felt manipulated into marriage and that his shotgun wedding and the price he paid for it weren't as accidental and innocent as they first seemed. Perhaps he felt as if Cynthia was "taking a ride" on his hard-earned fame? It's worth mentioning that Cynthia had her own fan club, the only non-Beatle to be so honored. So "hurt my pride like her" takes on many possibilities that don't necessarily mean that Lennon was actually having an affair. He was fantasizing it though, quite clearly: "If I fell" in this reading does not mean love.
So, I think this song is very deep for John, and certainly much deeper than most gave it credit for. After all, "she will cry when she learns that we are two," will cause (and probably caused) Cynthia great pain as well (when she heard the song). It's interesting to note also that the love interest he is singing to is very aware of his being married and that he is still in love with his wife ("if I love you too"), otherwise he wouldn't be telling her this whole story with such openness. John's sincerity comes through supported by the dual melody lines as mentioned before that reflect his divided mind. Yet, in his affair there is a duplicity and insincerity, that “being in love” doesn’t absolve him of. His pleading, as in his repeating the phrase "oh please" in the second verse, shows his increasing desperation. In the transition from the third verse to the bridge, he shifts the resulting "pain" he felt from the thought of his own hurt "pride" in the first bridge to the "pain" of Cynthia learning that he and his new love interest were now "two" in both of the remaining bridges. So guilt. Did I mention that?
Remember, “Pride comes before a fall”, and the title of the song is If I Fell. The second verse ends "Don't hurt my pride like her". Lennon already knows he is a slave to his ego and pride. That's the honesty: he's us telling the truth about himself and the affair, but he is wracked by the guilt of it. "If I Fell" is thus the contemplation of a willing transgression of "what is right". It’s very mature songwriting throughout and makes sense at many levels, both musically and lyrically.
[Edit: typo]