How to inculcate metta in me?

Metta was natural to me when I was young. I didn't give much thought to it then. Now I find it difficult. So many thoughts come when I want to donate to charity like, *is it a recognized charitable institution? how is it run? if they are adequately funded, should I donate to an institution which does not do marketing? or whether I should give the part to the homeless person?* All these thoughts makes metta a mental process of deciding who is worthy and who is not with limited funds I have for donation. How does one inculcate metta? Is it influenced by skandhas ? Can metta itself become an attachment leading to mental craving for that feeling when one can observe on giving?