It’s common to try to use a lot of effort in metta pracitce. Firstly to fix attention on metta ‘hard enough’ that attention won’t drift, and secondly to summon up the feeling of metta. But this approach inevitably results in frustration and a sense of failure.
When it comes to steadying attention, strong effort is the wrong tool. The mind is a complex system and not all parts of your mind will be in agreement about how to best use attention. Some will be trying to meditate, some will want to think about and do other things. It’s not a failure when other intentions take over - that’s just part of the process. So instead of trying to hold the impossible intention to have perfect control over the mind, which will always lead to negative feedback, just do the only thing it’s possible to do, which is to apply gentle intentions, and repeat these frequently.