Hi welcome to the forum!
If you try to force out thoughts of aversion, that's may just be another aversion: aversion to those thoughts. If you think you're somehow bad or wrong because you have such thoughts, you should cope with that first. Perhaps let aversion just be. Cultivating metta is a good method, seeing the downside of aversion also is, but letting be is another good method. The thoughts surely won't last forever. We all have them from time to time, it's human. And it's not always easy to steer the mind away from aversion. It takes a lot of practice to be able to do it in any situation. So it's ok if it doesn't always work. In the beginning it's hardest, but if you persist and don't act upon aversion, I assure you it will get easier.
I find MN20 (click)
the best sutta on this subject because it gives 5 methods to use, and in a specific order. I understand them as such:
1: cultivate metta / compassion
2: see the drawback of aversion
3: let it be
4: take away the willpower/delusion driving the aversion
5: tell the mind "no"
If you start with number 5, that may get you into troubles. It's only a last resort. But if you stick to number 1 as the one and only tool, that may not be enough. You can't cover up all wounds with a plaster, sometimes they need another treatment. So you could use this scheme of 5 methods, or you just realize you have to be a bit more creative. Because sticking to a method is another danger.
The mind is very subtle and to get a grip on such mind states is a delicate process, it's a bit like surgery. Sometimes when overcoming aversion you start another hindrance, for example. But with time you will get on a level beneath thoughts. There it will all be a lot clearer, and the list of methods above is not needed anymore because aversion just bounces off.
It could also very well be that "may all be happy" is too big an object to send metta towards. Pick someone or something easier. Perhaps a pet. And if repeating a sentence doesn't work, use some imagination. Really see the person or animal in front of you.
This is all based on my experience. All minds are different in a way, so see what works for you. Be creative.