Please help me i’m desesperate i’ve been for a few days for some stupid problems, first: i couldn’t implement any custom animations to a character i bought in the marketplace, i tried exporting it with unreal as fbx and then uploading it to mixamo, and when i tried to insert the character back in unreal it said that “there is no mesh information” i looked about it and THERE WAS NO INFORMATION OF IT holy ■■■■ it’s super difficult to find useful information about UE4, well, i heard about skeletal retargeting so i said hell yeah let’s import all the animations to a mixamo default character, then implement that character to UE4 and retarget all of the animations. The only problem is that all mixamo characters are T-pose and the character i bought is A pose, i watched a lot of tutorials and all said the same just change the rotation of the arms and that’s it. I worked for days, creating animations, montages, connecting it to the blueprint, everything was working fine, and when i tried retargeting the whole thing went to ■■■■, the character looked horrible, the head was super distorted, not a single animation worked, i couldn’t reverse that skeletal retargeting so i tried doing it again but changing the previous skeleton as the new one, it got even worse, now the head is just floating around and when i press play the character doesn’t even moves. holly ■■■■ this is so painful and NOTHING exists about this problem, i’ve beek awake for 2 days trying to figure this ■■■■ out and there is NO INFORMATION WTF
First off my dude, chill 
Game Dev is suppose to be fun
Secondly, sounds like you need some more experience with the export/import settings in UE4. As I’m sure you know, fbx is a very flexible file format that allows you to include skeletons, meshes, animations and other sorts of data. When you export / import FBX files, pay attention to the checkbox settings.
I would recommend watching a few videos to get a more fluid understanding of the export/import workflows.
No matter how bad things get, as long as you have source control, and the original files, you should be good. That’s general digital content creation 101 - always keep backups and always keep originals!