Jennver:
As you are using the spyder for profiling your monitor, you SHOULD be able to do better. But still keep in mind that the print is reflective and the monitor is transmissive. There WILL be differences. What you need to do is understand what they are and minimize them. You need to know what to expect also. Have you done much research on color management? This might be a good place to start. This wiki link has some information to get you started:
Color management - Wikipedia, the free encyclopedia
First, you do not mention what color space you work in, sRGB or Adobe RGB. Adobe RGB will give a wider color gamut and possibly less contrast. The system you are using, the spyder, should let you set the proper color temperature (usually D65 or 6500K is good for monitors, and not 5000K or D50, which is the standard for printed media). Also, you mention that the prints are coming out dark, but you increase the brightness of the monitor. To get a better handle on what is being printed, decrease the brightness of the monitor. This is something the spyder should assist you with also. Understand that by increasing the brightness of the monitor, you by default see more depth and tones than you may see in the paper. Finally, you do not mention how the prints are being made. If they are from your own printer, you should color manage it also. Depending on the model of the spyder you have, you should be able to do this. If you are having them printed off site, and you do not want to send them files to print so you can color manage their process, then you may have to just understand that you need to make the image look flatter on the monitor, and lighter, to get it to fall into place when printed.
Digital color management is a discipline all to itself. But with the right tools and understanding, you should be able to get great repeatable results.
Regards,