I think Sartre explained this when he proclaimed that we are "condemned to be free."

I'm pretty uneducated in regards to existentialism but I'll take a stab here just for the sake of argument.
Can it not be said that the determinist views human consciousness as a creation of the universe, whereas the existentialist views the universe as a creation of human consciousness? If so, they are starting from different premises and the two positions are not so much in disagreement as mutually incomprehensible.

At the level of Newtonian physics, the universe is causal, but there is still no solid answer as to if that holds up at the quantum level. It is possible that to some degree, free will exists as more than an illusion of consciousness. More importantly, it's kind of a moot point. If free will exists, we exersice it. If it doesn't, we still effectively exersice it, even if it's illusory. But personally, I can see what you mean about some people clinging to nondeterministic free will as a comfort, rather than as a rational viewpoint. I guess I could handle either fairly easily, myself. After all, if the universe is only mildly nondeterministic at the quantum level, to such a small degree that it is virtually impossible to see at the Newtonian level, then it's not likely to have a substantial impact on brain chemistry anyway, still leaving us (probably) without real free will. And I'm not sure I would call a cosmic RNG 'free will' in any meaningful sense anyway.

I'd put myself somewhere in a category like that. I think what makes this a difficult thing to really dial in on is being an existentialist is not easy. It's not like in some Camus book where the guy just is an existentialist with out really being aware of the concept. Some times I do have emotions that well up, you know? It's how you deal with them I suppose. Does it mean anything that i became happy, or sad, or jelouse? Not partoculalrly. I don't bottle emotions, they just evaporate when i let them be. Someone once described me as an 80/20 apathetic, and I think realistically that's as close as a sound of mind individual will get. For the most part, I remain unaffected.

As for the deterministic aspect, the key there is that nothing really changes. Not believing in liberatarian free will doesn't just turn you into something floating in the ocean with 0 power to affect your heading, it's more an understanding that there is no source of choice separate from the chemical mechanisms of the brain. It's a scary concept, but has reletively little impact on your day to day life. You will still decide to do plenty of things, the concept of will power still yields results. I just happen to believe that the feeling of that coming from a separate self is likely inaccurate.