Resistance to Loving-Kindness

Mindfulness meditation is four-fold:

MN10:2.1: “Mendicants, the four kinds of mindfulness meditation are the path to convergence. They are in order to purify sentient beings, to get past sorrow and crying, to make an end of pain and sadness, to end the cycle of suffering, and to realize extinguishment.
MN10:3.1: What four?
MN10:3.2: It’s when a mendicant meditates by observing an aspect of the body—keen, aware, and mindful, rid of desire and aversion for the world.
MN10:3.3: They meditate observing an aspect of feelings—keen, aware, and mindful, rid of desire and aversion for the world.
MN10:3.4: They meditate observing an aspect of the mind—keen, aware, and mindful, rid of desire and aversion for the world.
MN10:3.5: They meditate observing an aspect of principles—keen, aware, and mindful, rid of desire and aversion for the world.

Rather than skipping directly to the fourth step for metta meditation, proceed through all the steps in order, passing from one to the other at the right time. Your aversion might simply be to doing things out of order.

Please also note that right immersion is not forced:

DN34:1.6.69: Right immersion with five knowledges.
DN34:1.6.70: The following knowledges arise for you personally: ‘This immersion is blissful now, and results in bliss in the future.’
DN34:1.6.71: ‘This immersion is noble and spiritual.’
DN34:1.6.72: ‘This immersion is not cultivated by sinners.’
DN34:1.6.73: ‘This immersion is peaceful and sublime and tranquil and unified, not held in place by forceful suppression.’
DN34:1.6.74: ‘I mindfully enter into and emerge from this immersion.’

I have done breath meditation or many decades. I still do it. The other meditations come naturally--after breath is settled.

You can also be active like helping and teaching kids in buddhist monasteries, doing donations for food, shelter, reforestation, volunteer work...

If you aren't comfortable with a technique would should go very slowly maybe 1-5min a day. Maybe Tonglen or Mantras(Chenrezig) would be more fit for you. https://www.lionsroar.com/how-to-practice-tonglen/

You should ask a Guru a Lama or Monk he would know.

I would think of this as generalized aversion.

Aversion is a hindrance, one if the five hindrances. Whenever one notices a hindrance one should direct the mind accordingly.

If thinking about doing a certain contemplation seems tedious then one should resolve that aversion before doing anything else. One does this by recognizing the circumstances, properly reflecting and secluding oneself from the triggers mentally & physically. When a stilling of unwholesome thoughts has occured one can just enjoy that and not worry about whether one used metta or whatnot to attain it.

There is nothing repulsive about metta contemplation but contemplation or meditation in general can become repetitive & a chore. It is especially boring if one is putting in a lot of work but get's neither special rapture nor lights & visions.

How one deals with aversion is a matter of trial & investigation. One should learn many skillful means such as redirecting the mind to the body and the themes of appriciation & etc. One might think that metta counters aversion but if one is averse to doing it, as one has been doing it, then it isn't at that time suitable and one can choose to investigate the aversion or choose another gladdening theme such as ie compassion, appreciation, equanimity, virtue, The Buddha and whatnot. One can also do breath work to shut out distracting thoughts and thus create distance & attain seclusion in order to perceive the grand distinctions such as lights & visions.

Forcing oneself to meditate with an aversive mind is not a good idea but the stilling of aversion is a meditation and a good idea.

You can also try mixing things up to avoid monotony in contemplation, try different ways of rousing the mind and develop wholesome themes by giving of attention as you see fit.

How can this be explained?

That's aversion towards the meditation practice. Just note it as "disliking" and be mindful about it until it passes away. Be very vigilant in observing this aversion when it arises as it can sabotage ones practice if left by itself.

Eventually, as one gains more and more clarity of mind, this aversion will subside due to the mind understanding the benefits and freedom that comes from consistent meditation practice.