The time has come for mastering, but you've gone over your budget on the project. There is only one solution, you're gonna have to try your hand at mastering. This can be a scary idea, but not one that is impossible. Let me give you a few helpful tips to calm your nerves. This can be a simple and straight forward experience that will give you a deeper appreciation for your own music.
First thing is to determine where you are going to try and accomplish this feat. If you don't have a acoustically treated room, then use a room that you usually use to listen to music. A room that you're familiar with and can gauge the level of sounds with ease.
Second, give your mixes a little bit of headroom. Typically I request -3dB to -6dB of headroom on the final mix. This will allow you to EQ and compress to the level you want without generating too much distortion or digital clipping.
Third, start with subtractive EQ. Pull out the frequencies that are clouding or mudding up your mix. Get a nice clean tone to the mix before you start to compress or limit. Once you have a nice clean mix, try a compressor on the signal. Any compressor will do, but if you have one specifically for mastering, even better. Bring the threshold down until you see 2-3dB of gain reduction. Adjust your attack and release times so the compressor doesn't make the music pump. A slower attack time can allow transients to pass thru and retain a crisp dynamic range in the mix.
Forth, this is the most important part. Use a limiter that has a built in dither. Quantize to your desired bit depth. Most audio is dithered to 16 bits and this bit depth is optimal for CD quality. Be sure to use the appropriate noise shaping. Type 1 is best suited for voice over or narration. Type 2 is best for mixes that have multiple instruments. The final level is up to you, but this is why I suggest a room you're familiar with. Play a track that you want to use as reference. This will help you gauge the final volume level you need to achieve. A/B your master with the reference track and make your adjustments. It's really important to set your monitor levels at a modest volume. Don't fatigue your ears with loud volume levels. Keep an eye on the gain reduction in the limiter. My rule of thumb is less than 4dB of gain reduction is acceptable. Higher levels of gain reduction lead to hyper-compression and loss of dynamic range.
There will be some level of trial and error that you may have to overcome. Don't worry, you're new at this and the learning curve can be steep. You're not a pro mastering engineer, but you do have the ability to get a decent master with a little bit of practice. Give it a shot and if it doesn't work out, you can always save up to send your mix out to a pro mastering house. Just have fun and see what you can accomplish on your own.