You situation will crash into a dead end, leaving
you critically impaired.
Your boyfriend seemed to have mistaken the intent of your
confession - he sees it as a signal to propose a marriage, bringing
the relationship to the next level when in fact, matrimony is not
appropriate at the moment. Because he's almost lost you, he would
probably take haste to bind you into a marriage before your love
vacillate again.
Yet unknown to him, this man doesn't know that your love for him is
fading. His hasty advancement towards marriage is building on the
fact that he probably thinks that you still loves him very much,
albeit somebody slip into the picture. This is something quite dire
and do you even plan to talk to him about this to resolve this
conundrum?
I cannot advise you to marry him unless every fragment of your soul
is willing to be his wife because as long as you are not
emotionally ready, you are risking complication and frustration
into your marriage life later on. You are agreeing to his plan of
marriage solely out of duty... accounting this one decade of
relationship, which amounts to nothing if you do not feel this love
inside of you.
One good question: is it really because of this school mate that is
causing your feelings to waver or if it because as time goes by,
your feelings diminished insofar as somebody could easily slipped
into your heart and implant confusion?
There is a distinctive hairline difference between the two because
if the answer was the latter, you could jolly well be dissatisfied
with this relationship all these while, waiting for somebody
suitable to come by before you take that initiative or courage to
leap ship. And in this case, the catalysis is your school
mate.
In that case, I suggest you speak to your man and hold back all
plans first. All plans made in name of marriage must cease until
you have decide on some kind of alternatives, which enables you to
carry on in this relationship. Be it changes, compromising or
whatever - do you even feel enough to believe that this
relationship could probably work out because something is lacking
and you want to patch up those voids?
Of course, if you opt for the transgressive approach - in
attempting to reveal your feelings to your school mate AND see if
there is a possibility between you and him - be prepared for heaps
of emotional distress and suffering.
It's useless; even if your man give you the moon in the sky if you
do not feel like being in this relationship with him or having that
wanting to work things out mutually.
Love is freewill - when you remove duty, moral and social bondage
incarcerated by our society.
If it was the former, I would say that if you cannot handle the
implication of your overwhelming emotions for him while trying to
maintain friendship, I suggest you just remove having that contact
with him. If you cannot handle fire, you eliminate fire before the
flame consumed you totally.
It will grow if you allow yourself the condition to make it grow as
maintaining friendship is but a facade.
You have to make critical decision for yourself and not fly on
auto-pilot on this because if you do not take drastic measure, you
will crash into a dead end. And it will definitely happen, if you
do not take heed and jammed those breaks before you retrace a
route.
Drastic measure
must be back-up by concrete reason. Should you
decide on any decision, make sure you believe in the decision you
have chosen. 
Cheers
