I never considered self-sacrifice a form of suicide. Though they both fall under "suicide" by definition, they're almost totally unrelated acts.
Suicide as I define it personally is a selfish act of an individual unwilling to believe they are able to go on in life, and therefore deem it necessary to die, almost always for everyone else's "benefit", without once considering the grief and sadness they would put those same people they're trying to spare through.
Self-sacrifice is a willful act to preserve another's life or well being through an act of willfully placing oneself in harm's way, such as pushing one person out of the way of an oncoming vehicle only to be hit yourself. Though the same ramifications of causing a great deal of pain to the one you were trying to protect exists, the reason for the act is far more valid.
In short, under a blanket definition, yes, suicide can be both selfish and selfless, but it depends entirely upon the reason(s) for the act.
Your own personal definition is irrelevant. It's the actual definition that matters.
The Wiktionary definition is:
Intentional killing of oneself, as a kind of action or social phenomenon.
If you sacrifice yourself to save someone, you are clearly intentionally killing yourself, and thus committing suicide, though it is not selfish.
I honestly don't get how anyone can say suicide is exclusively selfish. Dealing with absolutes is meaningless.