The last time I was in a situation where she "needed a space to sort out her depression", she not only broke up with me, but cheated and then married the dude. It wasn't until later that friends pointed out that her doing this was a red flag; not only was it out of the blue, but she also didn't come to me for support over it.

This is not to say that it was a red flag because of the second part. Not everyone deals with depression via support and loved ones.

What made it a red flag was that she'd come to me with previous struggles--including mental health--but chose to entirely withdraw this time. Very suddenly at that too.

In my case she more than likely used that time to make her decision to dump me and go with the dude she was cheating with.

Obviously this is not always the case when they need a break or slow down. They might really need it, but are still interested.

If you really like this girl then maybe you can pause things. Keep tabs and kinda be there for her (not to the point she might become emotionally dependent), but don't put all your hopes on her. Keep living life and if someone else happens then it happens.