Im trying to wean my baby off the breast but it's prooving very difficult?

Hi all :-) Ok, I have a 9- month old baby girl, who i have ONLY ever breast fed her whole life, never even given her a bottle before! However, for various reasons, i would like to stop breast feeding her now, mainly because there's some medications i should be taking but cannot due to breast feeding. I tried to give her a bottle of Cow and gate follow- on formula, she had a hard time using the bottle and was not able to suck, so i have started using a beaker cup, she does drink the formula sometimes, but its just sooo difficult weaning her off the breast as this is her comfort- particular at bed time! Does anyone have any advise to offer, or share your experiences? I feel cruel but i really do want to stop breast feeding her now, i don't use a dummy or anything and would prefer not to. This is my first child. Many thanks in advance :-) x