Trouble with levels

Ah, the age-old question. In a nutshell:
 
-12dB is 1/4 the amplitude of 0dB, and power, which is roughly proportional to perceived loudness changes as the square of the amplitude. So, all other things being equal, a signal peaking at -12dB is going to have 1/16 the loudness of a signal peaking at 0dB. But all things are not equal because those commercial recordings you're comparing to are using compression, limiting, EQ an other tricks of the trade to increase the RMS power while limiting the peak amplitude.
 
So basically you need to need to use a combination of gain, compression, and limiting to get the peak up close to 0dB (no more than -0.3 is advisable if you intend to export to MP3), and raise the power level. And look into EQing out the frequencies that add a lot of energy without contributing to loudness in the range where humans are most sensitive to it.
 
But first, do some Googling on Mastering and Loudness Wars.

Sorry, I should have Googled MLP1 Pro before posting; you're use of 'peaking' threw me off. So I gather MLP1 is showing you RMS amplitude is around -12dB, which is typical of a signal that peaks at 0dB, and hasn't been absolutely crushed with compression to get the power up. The first question then is what choices are you making in the Export dialog? And if you import that exported file back into the project to a track that goes directly to hardware Main Outs, does it sound as loud in SONAR when you A/B against the live sound on the Master bus (group the track and bus mutes in opposition). If it does, check whether you might have pulled down the Main Out volume at some point, and were making it up with monitoring amplification.
 
When preparing to export, I like to bounce a copy of the Master bus to a 'Master Bounce', using source = Buses, that goes direct to Main Outs as described, A/B (or invert phase to check nulling) against the live Master bus to make sure everything is right, and then export that 'Master Bounce' track. An incidental benefit of this is that you automatically have a copy of your 'Master' preserved inside the project for future reference, and you can make additional bounces to new lanes of the 'Master Bounce' track as needed when you re-mix.