Hello!
I’m recording on two channels, two mics. Sounds quality is amazing.
When I’m mixing it down to Mono, it becomes all echoey and sounds terrible.
Why is this happening? What can I do to keep the original clear quality?
Thank you!! I’m at my wits end!!
Kasia 
One of the microphones is probably inverted/phase reversed. (If that’s the problem it’s a hardware issue. It could be the mic or the cable or some mixers have an phase/invert switch if you’re using a mixer.)
Click the little drop-down arrow to the left of the waveform (maybe the same method you’re using to make mono). That allows you to edit the channels independently.
Then select one of the tracks (it doesn’t matter which one) and Effect - Invert. Now, hopefully both channels are back in-phase.
I’m recording on two channels, two mics.
Any details of the actual recording? There are ways to make two normal microphones do that by application accident, but there are also ways to create a bad audio cable. The bad audio cable will keep producing bad recordings one after the other. It would be good to find it.
Koz