Other answers have covered My Guy Syndrome and misaligned game expectations. I would like to address one aspect of your question that I think is common in D&D groups, and that you as players need to address together.
Your Character Can't Walk Away
Your play group are probably friends out-of-game (or at least friends of friends). You all decided to get together to play D&D for various reasons. But, in-game, why did your characters decide to get together and have this adventure? What are the bonds holding them together?
There was one particular part of your question that struck me,
It reached a bit of a boiling point where we are in a pyramid trying to find a mummy lord to get their wraps for plot reasons. During a combat encounter, the bad personality came out for whatever reason and, lo and behold, started attacking the party again. After he came back to his senses, I simply told "Barb" (as a mix of both player and character, who were both fed up with this) that "I will not heal you until we get out of here. I will keep you alive, but until then, you will not be getting any support from me. No Aid, no Death Ward, no Cure Wounds, nothing." This was my way of trying to say "Now is not the time for this, and this is your punishment."
Other answers have explained why responding in-game like this may not be a good solution. But pragmatics aside, this feels like the response of someone feeling very frustrated and very powerless. And I think the reason is that all of your usual tools of social coercion have been taken away!
When you work with a group in real life, you have a whole toolset at your disposal. You have shared goals, personal history, and a place in a social structure that you each need to maintain. It costs social currency to be an asshole (at least an obvious asshole). But the biggest ace-in-the-hole you have in real life is that you can walk away. You can leave the group or quit the job. That's helpful because it gets you out of the situation, but it's also a source of power because, if people need you, they at least need to behave well enough that you want to stay.
So, if your character is in a party with someone who, half the time, turns on them and attacks, why would they keep adventuring with them? Why would they not decide to quit this group and go find a different group who is more aligned with their quest? That would be a very reasonable response in the real world.
The problem is, in this game, your character doesn't actually have the option to walk away. Because you, the players, are friends and decided to play together, you've all implicitly agreed that your characters are going to stick together as well. And so, your character has lost that very important option.
Think about the places in real life where bullies thrive: schools, hard-to-leave jobs, insular communities — places where people don't have (or don't feel like they have) the option to walk away.
And so, because your most powerful (and natural) tool for enforcing social norms has been taken away from you, you're left feeling frustrated and powerless. You of course have the option as a player to walk away from this game, but that's a very different response. (I won't say whether you should or shouldn't.) Assuming you all want to continue this game, I suggest that you all take a step back and really look at why this party still exists.
Why Are We Even a Party?
Parties of adventurers form for all sorts of reasons. You may be old friends, fellow knights, chosen by the gods, or all in the right tavern at the right time. The question isn't why your party formed, but why your party works. Because 99% of groups that form in the real world break up. Companies fail. Friends lose contact. Couples divorce. People that meet in taverns go home and forget each other.
A lot of D&D groups just “make characters” and then start playing together. Each character may have a detailed and complex backstory, but if those backstories don't actually overlap then, from the other players' perspectives, that character is nobody — just some rando they have no particular reason to care about.
If you want to have a long-running game, you've decided out-of-game that this is one of the groups that lasts — that becomes one of the premises of your game. And it means that, out-of-game, you all need to figure out why. Some of this may come naturally from characters' backstories. And there may be practical things like, “we don't attack each other,” but they may also be things like, “we all share a strong allegiance to the king,” or, “we've put aside our differences for the good of the party.” Some pairs of characters or small groups may share their own bonds, e.g., two characters who say, “we are cousins from a tightly-knit family.” As long as each character feels bound to the rest.
And then you all agree to play within these bounds. You're essentially giving each other social leverage to make up for the leverage missing from real life. Or, to look at it another way, you're fleshing out the complicated lives of real characters that naturally lead to social leverage. Each player must choose for their character to care about these things and to interact as real people.
You Can Pick Your Friends
One caveat to this is that some combinations of characters strain credulity more than others. A standard spread of lawful and/or good characters may take little-to-no effort to make stick. A party with a lawful good paladin and a chaotic evil warlock will take a lot more creativity and effort. A party with a barbarian that randomly turns on them may be toward the higher-effort end of that spectrum.
This is reflected in the real world. Groups tend to be like-minded in some way. It's much less common to find a healthy group of polar opposites — and when you do, there's often a very interesting and unlikely story behind why they stuck together.
One thing players love about D&D is the freedom they have with character creation. But when a DM gives players a lot of latitude in character creation, they're giving them a lot of power over the story, and with great power comes great responsibility. If someone decides to play a character that is difficult to get along with, you all, and they especially, need to commit to making it work. Or else they or the rest of the group need to change their character concepts enough to make it work — because again the fact that the group sticks together is a premise of your game.
So for the lawful good white knight of a paladin and a chaotic evil devil-worshiping warlock, maybe the paladin's player decides that they bend over backwards to see the good in everyone, or the warlock's player agrees that they keep the devil-worshiping on the down-low. Or maybe the paladin is turned into a neutral good fighter with the knight background, or the warlock changes alignment to chaotic good.
It will take creativity and probably some meta-gaming — which in this context is not a dirty word, but just reflects that the rules you've agreed to at the game level (the “meta level”) must affect the choices your characters make. Remember that real people are really complicated, and they make lots of choices that are non-obvious because of decades of hidden experiences, fears, loves, and desires. That gives you all a lot of room to work in to stay “true to the characters” while at the same time making concessions to the game and to realistic social relationships.