There doesn't seem to be many responses that address this part of what you wrote, so I hope this reaches you well. Let me try to rephrase it to see if I'm understanding it. It's that you feel like the absence of vocal inflection and non-verbal cues lead you to interpreting Slack messages in a more negative light than you feel you should be? And this is adding up, yes?
If I'm in the ballpark, I might suggest chatting with a therapist. You sound like you've identified the distal cause, but understanding the root of the issue is going to be instrumental to identifying the necessary changes to get you on the right track. While I believe the responses you have received are well-intentioned, it's my belief that you'll likely make great strides by talking with someone who can engage you with real conversation and have a back and forth and cooperative approach to figuring this out.
I hope that no matter the path you follow, you end up working it out. It sounds like it's impacting your life in a pretty big way. Take care