The Nuances Behind "No"
You thought that someone was made for you, everything seemed to fall in place, conversations turned deep; but then, you have lately realized that this person isn't for you. You now wonder how to say that "no."
Sadly, the world finds the one initiating the separation as egoistic and wrong. This need not be so, when the decision to part ways is to safeguard the lifelong peace of the two involved. Wedded life is the beginning of ignoring that individuality and shunning that selfishness. Thus, if you want to leave that love behind with a true sense of inclusivity, you really need not feel that guilty again."
Hope you are lighter now, and the question is how to get the other out of your way, shattering them the least. If the other is pretty much mature, then this is no issue, as they know to move on and get ahead. But if they have tangled up with you quite deeply in a relatively short span, you ought to make sure they are soon peaceful. So, you must make every move weighing your decision and their peace at par.
First things first. Be truthful about why you wanna back off. Lies won't work as you expect them to. Always remember that one harsh truth is way better than a good bunch of white lies. Yet, don't get into detailing the problem. If the other has grown so fond of you, the madness might force them to lie that they would walk the last mile. This would needlessly complicate you both phasing off forever. Of course, promise the other better times as a friend, if the parting is understood and accepted.
Well, do not resort to threatening at any cost. They don't deserve such treatment for the way they still hold you. Differences that cannot be played down have you here. The other will take time to grasp this. Until then, be calm and conscious to not hurt them more. Act with absolute awareness, be firm about the stand, and assure them that they will surely be happier about this decision later.