I translate the OP's "Is lying immoral" as "Ought people lie or not". If the question is about a specific instance of lying, then I've misunderstood what we're talking about. I was discussing idealized behavior. Lying can't be UPB, just like theft and rape. Unfortunately, the logical implication is that any should-claim that starts out "People should lie..." is wrong. If all instances of rape are wrong, each instance of rape is wrong. If all instances of lying are wrong, each instance of lying is wrong. Of course, Rape and theft fail UPB because they can't be universalized. Lying can be universalized, but it is incompatible with argumentation. Additionally, Lying is not objective or consistent. It fails UPB for different reasons, but it still fails.
As far as not wanting to tell someone the truth... Sometimes I lie to people. When I do, I stop and tell them I just lied (I dislike lying). I then use that as a springboard to discuss my relationship with them, with the goal of discovering why I lied and remedying that problem (i.e. I just lied to you, why do you suppose I did that?). Lying is either a tool for control or a defense mechanism (or possibly both). If I lie to someone, it means I'm uncomfortable with some aspect of our relationship. Generally, I'm either scared or I feel threatened in some way. By facing my choice to lie and understanding why I took that action, I can better understand the terms of my relationships with that person. I'm of the opinion that, should you find yourself lying to someone, it is a clear indicator that there is something wrong in your relationship to that person. However, the morality of lying and the relationship dynamics of lying are (at least in my mind) two entirely different things.