I think a lot of the time, the problem is psychological in nature. You spend a lot of time and effort getting in, they just swat you away, and maybe after a few iterations of this you start feeling frustrated and go on tilt. Mostly what you need to do is take a step back and do something like the process below.
1) What am I doing wrong?
2) What are my best options?
3) How did they react when I did get in?
You can also sort of break it down into a move + countermove dynamic. He rolls, I Down B. He Dsmashes, I block. He holds shield, I grab him. He spams something I don't know how to counter, avoid situations that allow them to spam it.
When you think on smaller terms like this it can be a lot more doable in the heat of the moment. It also helps to know that in general, like 2 successful hits from you = 3-4 for most other characters.
In terms of actual gameplay tips, I would say try to stay within Down B range, but outside their range (you outrange probably every dedicated projectile user's normals). That is probably your most effective range and a lot of habits become very noticeable once you see what they do once you are there.