and it kind of confuses me because I feel like there shouldn’t be THAT many people who struggle with mental/emotional issues

But just because you think that it doesn't sound right doesn't mean that it isn't actually the case. This is especially the case for those issues which aren't always easily observable, like depression.

but then why is it becoming more common just now, during this time where almost everyone uses the internet?

Firstly, correlation is not causation. If there was a connection between screen time and likelyhood of being diagnosed with ADD/ADHD then it might also be the case that people with ADD/ADHD are drawn torwards using devices, not devices causing ADD/ADHD. Those are contrary explanations. It takes empirical results to decide which we should prefer. It might also be the case that there's a third variable.

Secondly, the premise that there's a correlation might be wrong. You haven't given data on the number of diagnoses. It might also be the case that screen time isn't rising anymore - here, we'd also need data.

Thirdly, we have to keep in mind that we can also explain an increase in diagnosed people as improvement in diagnosis techniques. The label ADD/ADHD was only introduced in the late 80s. A possible explanation that goes a similar way is that with reduced stigma of mental disorders, there are less people that won't get diagnosed because of it.

My argument is that your reasoning is incorrect. Your approach needs to rely on data to substantiate the claim. This does not mean that your claim has to be wrong. But our opinions on such matters should depend on our justifications. The justification that you've described here is too weak because it lacks reference to any empirical studies, or something similar.