Often when talking to many of my friends, they often try to put themselves down by commenting that they are not "amazing" and that they are not good enough. Often persuading them otherwise is a futile attempt, sometimes even causing an argument and making them feel worse (and potentially damaging their relationship). It's also worth noting that these conversations take place over a messaging service, and physically visiting them isn't exactly a viable option.
Is there any way I could approach this issue to help them feel more self important, or feel better about themselves, or is it better (and easier) to simply not discuss the matter further to prevent an argument.
To help you better understand I'll provide 2 examples of this, please bear in mind I am trying to get a general response to the question, rather than a solution that just fits these 2 examples (however feel free to use these 2 examples to show how I should reply).
Friend: Nuuuu I look ugly
Me: No you don't what do you mean you look amazing!
Friend: No I don't
Me: Yes you do!
etc... (this goes on for however long)
Friend: I'm actually pretty weak, useless like I am at most things
Me: No you're not, you're great!
Friend: No I'm not (inserts generic excuse about how they have a negative impact on those around them)
and then the conversation goes downhill from there...
It's worth noting that these are not "humble brags", they do genuinely mean this.