its natural, and entirely unavoidable. The spark in every relationship fades eventually. There are no exceptions, only liars.
What happens is that most poeple have relationships, then they end them when the romance fades, and seek that new romantic feeling again...
that is until they reach a certain age. Its the age when worries start to kick in, and they start to ask questions: Am I to old to find a partner better then who I have now? Will dating in my 30s/40s be a nightmare? If I take time to find a new partner, will I have time to have children?(for girls) Will everyone think I am pathetic for being a single 30/40 something? Do I have the energy to be dating again? Will the next guy really be better then him? etc. etc.
At that point, when they start to loose the romantic feeling - they don't seek to find someone new, instead they commit further to whoever they are with. They marry them, have kids with them, and start a family with them. Bits of spark will come back over time, and then go again.. but it will never be like a new relationship. Instead it will be replaced by a different kind of love - a longstanding family kind of love, where you can't even imagine not being with them, and they become as irreplaceable as your mum or your sister.
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Your 18. You don't have these worries yet. Why not start a new relationship? You have 10 years at until most people start to feel the pressure that this one, must be The One..
So why not end it? Enjoy the romance now while you can - make it so that by the time you eventually do settle down forever, you have the comforting thought of 'yeah, I had some exciting romance filled days, and now its time for something different'