You might want to check the control room mixer (devices menu at top, control room mixer) and ensure that the outputs there aren’t over 0.00 db.
If I have this right (apologies if I’m wrong)
In a very loose manner of speaking, when your mixing multiple tracks, you kind of fill a bucket with sound.
Your Master output is a bucket of sound, but it’s being filled with multiple full buckets from your multitracking…
So, you tip a bucket of sound into the master output that you’ve got close to 0db. You keep adding full buckets, well, the master will overflow - not in a simple addition scale (1+1=2), but on a logarithmic or other inflated scale.
Solution is to mix your tracks to taste.
Play the song to the end to work out the maximum effective output level of the master channel, and take the master volume down so that the effective output level is just under -0db
If you send it off to be mastered, you will want to take the effective output to -3db (you want some headroom left for mixing)
If you want to attempt to master it yourself…
Set the effective output level to -3db
Mixdown to wave
Open up a new project (or a master template)
Import the wave
Add a Multiband compressor on the track, fiddle with it (or use presets) to get your best sound.
If you had a mono mixdown (personally, I track in mono), add a stereo enhancer - fiddle with it to taste.
Insert the 32 band eq - (or eq of preference) and check the different mastering EQ’s. (mastering typically boosts lows and highs to make up for the difference between how speakers pump out sound at low and high volumes)
If your that way inclined, add a maximiser. The steinberg one does fine without presets, but there might be a particular preset which makes your song even better.
Add dithering to the master output channel
Mixdown your mastered track.
Watch your gainstaging. i.e. make sure that the last plugin on your master track is under 0db when you’ve played the song through, if you’ve added the maximiser, make sure that the plugin before it is under 0db.
The maximiser output set to 0db will set the volume coming out of it at 0db (after fiddling with phasing or harmonics or doing some such magic). This is fine if it’s the only active track, if for some reason you try to multitrack your master, you will have the same problem of filling a bucket with multiple buckets as at the start of this reply… Keep your master to one (active) track, do all of your mixing in your mixing project.
All the best with your tunes…
On your mixing project, if you have any plugins on the output channel you will have some gainstaging to account for. (simple, don’t do it - if your EQing the output, what your probably trying to do is master, so leave the mastering element as a seperate project).