For one, actually becoming more relaxed in confrontations is not something that has been achievable merely by asserting that I want to be -- instead, the only thing that seems to work is being in confrontations enough more often to reduce the stress level of such an environment. While this works to some degree, it raises the anxiety or stress level of the rest of my life, and is thus completely antithetical to most of the other things I do to make my life better. If I can instead just avoid situations which raise my stress level, and in which I would perform less well, that seems more conducive to overall happiness.
For another (and this was somewhat already implied), making changes to one's personality often has other effects on one which may be undesirable. Being more assertive might make it more likely that one bullies others (unintentionally). I've noticed this in myself as my assertiveness waxes and wanes: if I'm feeling especially confident, I'm more likely to say things without qualification, and say things that I would normally stop myself from saying. Later, I wonder if what I said came across as asinine, since that's how I would take such statements.
I sympathize quite a lot with the original poster, because while I'm a pretty good developer (at least, some others praise my skill highly), I'm really, really bad at interviewing, sales, talking to clients in a situation where I actually depend on their response, etc. (A previous commenter was quite right when he compared interviewing and sales -- they raise the same confrontational anxiety). When hired for a previous job, I was told months after the hiring that the interviewer was quite astonished that I'd turned out to be an intelligent and very capable developer, since I'd interviewed terribly, and the only reason he'd hired me at all was that he was having a fight with HR, and had announced that he would hire the next person he interviewed to show them how that worked out. That worked out poorly for his argument, but well for me and the team. :)
