I agree with what @thorninmud said about metaphor. You have trouble finding the words, (or at least I think this is why, because this is why it’s hard for me,) because emotion is way more complex than “I’m happy” “I’m sad” “I’m angry”, and you ‘re knowing that. So instead of searching for words that convey the emotion, find something that can represent or evoke the emotions, and describe it instead.
Or: When you’re feeling, joy, say, what does that experience feel like? Are parts of you tingling? Feeling like they’re floating? Does everything get sharper, clearer, or soften at the edges? And when your sad, where do you feel that? In your chest? Your teary eyes? Does it consume your body? Try switching your focus slightly, and maybe that’ll help, is what I’m trying to say. Instead of describing the emotion, describe what it does to you?
Also, maybe it would help to try to express them in a different medium, at least to begin with. Sometimes something that doesn’t fit in words would fit in a picture, or a melody, or a movement, etc etc. Maybe that will help open the door to other ways of expressing it, and when you see it outside of you in another form, perhaps the words will come then?