Constant mindfulness hard to achieve

I find very hard to be always mindful of all activities (four frames of reference). At the beginning of practice, my persistence are aroused and gradually it becomes *indifference*/lax. Basically I am starting feel stress trying to be mindful..? Since it is necessary to have other factors like joy, calm (7 factors of enlightenment) to achieve wisdom, I assume I am doing something wrong. What I have learnt from sutta is 'sati' dhamma is cause for arising of other good dhamma. So what are the conditions to have this 'sati' ? .