After playing a few matches in MVC3 with some people way better than me they told me that my biggest problem was my inability to block.
How can I overcome this… Blocking is my weaknes…
Go to training mode and record what the person was doing that you couldn’t block then see what you can do to counter it.
I’m not an MvC3 player, but I suppose it should work here the same as it does in SF.
Just go find a friend and play “defense challenges” with him - Endless Battle and one guy just attacks and the other just blocks and tries to stay alive as long as possible by blocking. Interchange roles after one match.
Other notes: Don’t hit random buttons.
Training Mode…
Set dummy to CPU…
Hold back as best you can…
Stop attacking! You cant block and attack at the same time as I’m sure you noticed.
Being decisive with your attacks as Alus said is the most important “first step” to building a solid defensive game. If you are decisive with your attacks, it means you mash your attacks less, which means you will break a solid block with a mashed attack less. Be patient and wait for the opening, but don’t mash out of your own block so eagerly.
It takes a bit to develop a good feel for it so don’t be too frustrated if it doesn’t come immediately! Marvel is such a fast game that attacks are absolutely going to slip past your guard, so don’t feel too bad about attacks slipping past your guard, just make sure they aren’t combo starters and that you can trade punches back to keep the game on even footing. 
Don’t play the computer, the computer can’t outthink you.
Play a person and have them come at you with everything they’ve got and don’t attack AT ALL. The longer you just block the more you will see patterns in your opponents play, you start to recognize start up animations and the game will appear slower.
Once you have a solidified defense start punishing mistakes and then back to full on fighting.
This should take a while not 5 or 10 minutes.
Go into training mode record the dummy to do what you had trouble blocking. Keep doing it until you pin-point where you keep screwing up (he keeps getting me with an overhead, can’t react to those cross-ups, etc.) then keep trying to fix those problems. Also if you’re in a game and you get blown by whatever mix-up or attack string while you’re getting combo’d or even after the match entirely, try and think about what exactly he did and how to defend against it.