So you would consider telling God you hate him a grave sin even if technically you don’t really mean it?
Objectively yes, it is grave matter.
Subjectively (as in you) - that would depend if you
knew it was a grave sin
before you told Him you hated Him. If you
did know, then it would be in my opinion a grave sin, though there
may be circumstances that make it less than mortal, or there may not, but that is something to ask the priest in confession, I would think.
As others have pointed out, there is a difference between just being angry with someone and actually hating that person. Only you know whether you actually hated God (at that time of telling him). Or whether it was something you just in anger. For example, if you had an argument with your boy/girlfriend and said “I hate you!” - out of anger and hurt and didn’t mean it because you do still love them but said it perhaps to hurt back, I would think there would be less malice compared to saying “I
hate you!” and actually mean you hate that person, can’t stand them, don’t want to have anything more to do with them, can’t stand the sight of them -perhaps this’ makes you sick’ or ‘makes your blood boil’, couldn’t stand to be in the same room as them etc etc.
I too have been frustrated and angry with God over the way various things are in my life, and confessed just that.
If it was me I would take it to confession and tell the priest what I’d said to God and all the circumstances etc. - but that’s just me.