I'ma discuss how to get over several general problems
Tunnel Visioning (AKA Forcing)
You just need to calm down. It isn't hard to hit people in Brawl, but people at a high skill level will be playing more cautiously when they are about to die. They will tend to shield a lot more, so watch for that and you can get free grabs. Remember, if you keep hitting them, they will eventually die from moves that aren't designed to kill. I have kill PLENTY of times with JUST U-tilt or F-tilt, or sometimes get a jab U-smash.
Getting Flustered:
If your opponent has the momentum, slow the game down. Run away, shoot jolts, play the spacing game, hell, even go to the ledge. Momentum is the scariest thing in this game because people will act more nervously and choose worse options/react to something they shouldn't. Just calm down and realize that you can switch the situation back with one read, but again, don't try to force it.
For Match-ups, all I can say is practice them. Get used to their openings as a character (Like Marth's weakness below/in front of him. Abuse their weaknesses and remember what can punish what. If you aren't sure, don't do it. Go for the safe D-tilt or something that is difficult to punish. Low-risk Low-reward punishes are amazing. You don't have to always go after them with an F-smash or a Thunder. Take your hits where you can get them.
Forcing grabs (This one is a specific issue since we have CGs)
There is, in most match-ups, no point in forcing grabs. You are trying to get early % on a CG, right? Well, if you camp your opponent and they get hit, you are getting the damage that you would get from the CG anyway, which is why I don't mind if Snakes blow themselves up at the beginning. The 30ish % they give themselves is 30% more on my side, and you can hit them off the level and edgeguard. Damage is the key, it doesn't matter how you get it.
If you have any specific questions, please ask.