this doesnt sound right. by 18 months you should be back playing sport. i assume you have been discharged from physio now but it sound like you could do with an assessment.
do you have full range of movement?
does the leg give way or lock?
any swelling?
where was the ACL graft taken?
did you have an associated injury as well as the ACL? ie cartilage damage as this may have deteriorated or require further attention.
i would see you GP or a physio for a full knee assessment. you may have to be referred back to your surgeon.