New to metta meditation I have some questions about metta meditation. 

1- When I send love to myself, my cat, the world etc. Do I have to feel something? Do I have to sit and repeat the mantra May I Be Happy until I feel love for myself and not move on to my cat until I've reached that point? (If yes, what if that takes forever? If I don't feel a thing, why might that be? Do I move on anyway? How much time should I spend on each thing?)

2- When I imagine myself, my cat, my neighbour and so forth, how much energy should I put into visualizing these things? Do I need to see it all clearly or is it enough to keep them in mind?
The way Trungpa Rinpoche explained metta, you must start with yourself. Sit and love yourself, respect yourself, give yourself a break - until you feel a relief. This may take days, weeks, or months - do not rush. The goal is to recover your sense of your own basic goodness, your fundamental sanity.

Often we judge others because we are strict to ourselves. Often we are strict to ourselves because our parents were strict to us, or our early environment was, like school teachers or peers. We may feel like we are high strung, we are under pressure to deliver excellent result, we are afraid to be a failure.

It takes a lot of courage to surrender, to sink into your own suchness, whatever it is. When you do it right you can feel the changes in your body. Your breath should get deeper and calmer. Your body should sink down and get more steady. At the same time you will feel like you can move your body freely without loosing your balance. Your face muscles should relax.

It feels like you can almost have a conversation with your own heart. "Forgive me my heart that I was so tough on you all these years. I'm so sorry, now I know how painful it was. Thank you for staying alive through all that. I love you, together we are one". You don't need to actually say or think any of this - but this is the feeling the right metta gives. The feeling of warmth.

This is 75% of the work. Then from this warmth, from this acceptance of yourself, you expand to accept and give warmth to everyone else. This is like having the same Sorry-Thanks-Love conversation with your cat, relatives, friends, strangers, enemies, and then the rest of the world, back to past ages, and forward to future ages. You don't need to visualize every one of them in great details, what matters is the sense of connection, the sense of appreciation, the sense of mutual acceptance, validation, and support.

It is a very heart-warming exercise. Don't be surprised if you find yourself overwhelmed with emotions and sob, that's a good sign.