I believe the problem is with your trigger assembly. Did you just put in a different trigger? I've had that problem in the past. Using one of the available closers mentioned in the previous post won't solve the problem.
I haven't changed thr trigger assembly and I haven't shot the gun very much. I see below that other 391 owners have the same problem and the easy closer solved their problem. I guess I will have to get one. It looks like Beretta would have engineered a better system.
If you're a "do it yourselfer", you can install almost any kind of flat bar over that button. Anchor the bar to the side of the receiver with two-sided tape. The larger/longer bar will give a larger surface for you to push against with your thumb. It will still take the same amount of total force to push the button in, but because the force is distributed over a much larger area, it will seem to be much easier to push. I did this on a 391 that I own.