How to inculcate metta in me?
If funds are limited and the opportunities to donate are plenty, you should find out which is the best place to donate as in who is genuine, who will benefit the most, who is the most worthy etc. It's an aspect of wisdom which guides Metta to have the best possible outcome. 

It's important to note the difference between worrying and looking for the best available option. The former is a consequence of clinging whereas the latter is a form of wisdom.

The Skandhas(5 aggregates) is a different teaching which has nothing to do with Metta, except for Metta bieing an instance of Sankhara-skandha(mental formations).

Metta can never be craving, but craving can disguise itself as Metta. 
Ex: Wanting to donate because the collection girl is pretty; wanting to join community service to get a chance to spend time with someone you are  attracted to; wanting to donate to impress someone; wanting to donate to get a heavenly birth etc.

Technically speaking, wanting to help the poor &amp; the afflicted is more Karuna(compassion) than Metta. Metta is simply kindness.