I believe it is selfish in a sense but whether or not you do it is still your decision to make. Like kahlann said, You're causing your loved ones pain to end your own. You know exactly what you are doing, who it will affect and what the consequences will be but at the same time, people cannot live on others' love alone and I believe that is where the line becomes a little blurred. I think it is selfish but not necessarily wrong or right, no more wrong or right than someone's fondness of the color green or the taste of onion rings.
You're basically saying "Well, I've had about all I can take and I refuse to take any more. So, see you guys later." So I think what it boils down to is what would you prefer, your suffering or your loved ones' suffering. It's a pretty vast topic as there are so many different scenarios and circumstances surrounding the motive for suicide. You can't really just slap 'selfish' on the act and call it done, not unless everyone had the exact same reason for feeling suicidal and the same people supporting/oppressing them.
But personally, and you can take this any way you like, I would not support the decision. I can't give a clear cut reason as to why, maybe it's just the basic instinct to survive, but I cannot see myself supporting someone's decision to kill themselves and then after the act has been done, patting myself on the back saying "You did the right thing."