Regarding #1, "Does the FBI/NSA or other government agencies have access to my data or can easily decrypt them?" legality aside, there are several factors to consider.
"Easy" is relative. Depending on the charges, and the value of the data that is encrypted, they may have more or less incentive to break the encryption. If it's a list of neighborhood junkies, they might not care as much as if it's a plot to bribe a senator and may identify an offshore bank account with $20 million in it.
The algorithm and keysize of the encryption you are using matters to a certain degree. If you use a 40 bit exportable DES key, it will be easy for them. If you used a 256 bit AES key, they probably won't try to brute force it.
The tool may be part of it. If you're using TrueCrypt, it might be more secure than if you're using ZIP file encryption, or passwords on Word '98 documents. Different tools have different vulnerabilities.
The time of the arrest may have a lot to do with it. If they suspect someone of having important data encrypted on their computer, they may wait until the suspect goes on line and types in their secret key, then knock on the door and seize the equipment while it is still running. They can then take an image of the RAM from the running computer, and possibly scrape the key from it. Most people won't think to clear their machine just because there's a knock at the door; or if they suspect the person of being paranoid, they might even go the whole "smash-in-the-door-Hollywood-style" route. Even local police agencies have "mouse wigglers" which are just USB dongles that act like a mouse moving every few seconds; they keep screen savers from activating while they wait for the digital forensic analyst to arrive to image the machine. You can buy a six pack of them on line for about $40.
Following on this, the law enforcement agencies also know that most people do not have good operational security. People write passwords on scraps of paper, or in a notebook locked in a safety deposit box. The suspect may have a password manager in their browser that reveals passwords for all their online accounts, knowing that people often reuse similar passwords online as they do for securing their data at home (if my password for StackOverflow is sekret$SO, and my password for Google is sekret$Goog, they might try sekret$TrueCrypt or sekret$TC when attempting to decrypt files on my home machine.) The FBI may run the NSA-provided equivalent of John the Ripper, which is likely a whole lot better than anything we might imagine. The suspect may have emailed the password to a co-conspirator, leaving it in their "sent" folder. If they can identify an acquaintance of the suspect who knows the password, they can compel that person to tell them.
Finally, if the suspect is really high value, Snowden revealed the NSA's infamous ANT catalog of tools and techniques they use to hack into running computers. If they can get a keylogger installed on the suspect's computer before they arrest him, they won't need to crack anything.
There are a lot of ways of accessing the data that are a lot easier than cracking the encryption. It all depends on their perception of the value of the data.
As far as #3, "Is there a better way to protect my data?", there are simply too many variables to answer that question. For every OpSec vulnerability I raised above, there are more that aren't listed. For every encryption algorithm you choose, there is an unknown capability of attacking it.