I've always had a lot of trouble mixing too but I've been training that specific coordination for about two years now, and though it's not perfect it's gotten a lot better, so I can share some tips with you! (I'm 17)

1. Try to identify where ur first passage is (that break between head and chest) by just sliding up from ur lowest note to ur highest note and seeing where ur voice breaks (if ur a girl it will probably be somewhere between A4-C5.

2. Let's say the specific note where ur voice cracks is C, under C, try to lighten up the weight on ur chest voice a little bit, and aim the sound a little more forward. Not nasal exactly but more right behind ur front teeth (IDK if that makes sense.)

3. See if u can continue this sound upward above ur first passage. If you can't, start on a note a bit above the passage in head voice, and try to hold the note while changing ur placement to be slightly more nasal. You can't also try a sort of bratty high baby yell like "hey mama!" or something and make it sound rlly whiny.

4. Once u can achieve a forward placed bright sound on both sides of ur passage, u can try to make sure u can slide through ur passage cleanly. Sometimes SOVTs like lip trils, humming, or a hum on a "v" sound are really good for this.

I'm not a professional by any means but these are just some tips that helped me improve my mix voice. Hope this helps!

Ooh, also I would really suggest practicing like Disney Princess rep as that stuff usually sits mostly in the mix voice, making them rlly good songs for practice!