Ok. Going to be a little different in this response so i don't mean any disrespect or anything by anything i say, i type from my head in here lol
Are you sure you want to be with him then? If you doubt you want to be with him then that is usually the answer. You are worried because you are concerned about being all alone or potentially seeing him with someone else? If this is the case then push past it. I was with my best friend for 3 years, inseparable we were, she doubted me, now she is gay and living the life she probably wanted and i have the life i would have wanted. Her doubt ended up being spot on about it, not that it helps my view of the whole thing.
So if this lad was the one you wanted to commit to and go off and be married to and stuff, you wouldn't doubt it as much. Trust your doubt rather than question it, if you know what you want deep inside then you will see the answer. Sparks do last, they form a very important part of a long term relationship when you look at that other person 30 years later and still see the same person you fell for.
What you want is the attention you feel like you deserve and you aren't getting that anymore from your BF. One question i have there is, why do you think he isn't paying as much attention as he did before? not every relationship is built to last, sometimes you put it down to a good couple of years and part ways. These things happen.
Disclaimer, the above is hopefully trying to play the other side to any responses you get. I don't mean it in a harsh way whatsoever and if i am wrong then disregard anything i have said. But i think perhaps it might be helpful to get told the other side of the story and then make your own mind up?