I had the privilege of occasionally teaching metta meditation and I used to ask people to remember how they felt when someone opened the door for them, or let them in during heavy traffic or smiled and said good morning to them - perhaps at the supermarket checkout. And/or how they felt when they did such things to other people.
Metta is actually a big part of our daily lives. We just don’t notice it. Partly because we don’t value it when it’s simple and humble - we want something more. But it just starts with recognising these simple feelings and “making much of them” as it says in the Suttas…i.e. beginning to value them and growing the habit of giving them a bit more attention, instead of the neglect we’ve cultivated towards these humble, pleasant emotions.
Basically, we don’t need to start off experiencing copious amounts of bliss. My philosophy is that if I can feel even a tiny little bit of metta, I can do metta meditation. If I can open the door for a stranger or smile at someone and say good morning, all I really need is to notice - and later remember - how I feel at that time - how this action makes me feel emotionally/physically - and use that feeling. From my limited experience, I’ve found this is all most people need.
It reminds me of how Ajahn Brahm teaches. He suggests you use a real/imaginary object to bring up the emotion of metta first. He recommends something imaginary in case you have complicated relationships with real beings. He even suggests the use of a teddy bear! 
I find anything that makes me go: Awwwww… 
…and can generally make a person feel a bit “melty” is of use. 
Wonderful topic and question. Anumodana 