if you really think that there's no more chance for your love for that person to grow and you feel like you really suck being in that relationship, i think the best way is to let go. even if it huts, its still the proper way out. Yes, i agree that letting go and moving is really SSOOOO difficult. especially when you come to be sooo attached and so in love with your partner..
but if you have passed through the stages of letting go and moving on,,, you'll be happy,,, and you might suddenly find meaning with all those pains you have gone through...
there's always someone out there who can be right for you... so don't think that someone whom you really love now but keeps on hurting you is the only person who can love you.. in the first place, if he/she really loves you, there's no way he/she will do anything to hurt you right?
so,,, always hope for the best... if you decide it is the best for the both of you to just let go and move on,,, then DO IT!!
no matter what happens after, be it regret or relief,,, its your decision and you have to face it.. there's always pain in this world but there is happiness also... don't be so trapped in your misery,,, there's always the SUN after the RAIN...
Take care ALL!!!