i must give the forewarning that i may turn tail and run, letting myself partially suffer and remain confusd...so any great advise is extreamly, greatfully apreciated, but if it is waisted on me, please tell me...

to shorten a very long story, i have a friend and i was pressured, by many events that happened, to be "theirs" (yes, i mean in the 'hook-up' 'really like' kind of manner, sadly)
before the quesiton was asked, this friends mental state had been worse than shakey, less than fragile...i'd do anything to cheer up my best friend, and now in doing so i'm going to end up hurting them.
they were having a very, very rough couple of months, and they'd admitted to me that they "loved" me (which, at fifteen to me is an unreasonable and even scary word when said its that context...)
that day was an expecally bad day for them...they asked if i would be "theirs" (this is all over text, if anyone even cares)and after i asked they admitted it would be extreamly painful if i said no, and it would help them through if i said yes. (what was i suppose to say...?)
[they had admitted they "loved" me earlier before, and all i was able to say was i don't know, but internally i pretty much knew i didn't feel the same]
i was confused at the moment if i returned the feeling but now i'm pretty sure the answer was no if i was truthful...and well, i said yes to them as a last ditch effort.
nothing else was working to cheer them up, so...it seemed the right thing to do.
we just passed our "four month anniversery" a few days ago...and i'm almost sure i don't return the feeling still...
is not for any physical reason, their very pretty, talented at drawing and writing, and very intresting... i just...don't feel the same...
(they have claimed themselves as a "romantisist", so thats why any of this even matters that much...)
i mean, there isn't much p.d.a, but...i don't return the feeling, don't want to hold their hand, don't want to do any of that...
i felt i had to return "i love you" and other things that normally people would enjoy in a "relationship"...
i'm naturally not a very gushy person, i mean, i didn't even want this kind of thing in high school...its just not what i wanted...
it souds selfish, and i know it is, god i know, but...what is the cure here?
how do i end this, without hurting them?
the only reason i'm even thinking about trying to end it is because...
1. I'm pretty much faking, and the mask tends to slip off very easily
2. when they find out i'm faking, its going to hurt them, so the sooner its disperced the better...their living a lie, and yes i feel guilt over it
3. in my mind it just complicates everything...
and in all of this...i've felt i've lost my best friend, someone who i could tell anything...is not the peron i'm hiding the largest secret from...

so what i'm asking for...is a way to end this without hurting them...
"its not you, its me" is so very true here, but hearing a cliche' will hurt them more, and thats the last thing i want to do.
so...please help, if you can or want to...