I've got a couple of these helmet liners and I really like it. The main reason I got it was to cover my ears so they don't fold over when I put my helmet on.. that was really annoying me when I first got a full face helmet.
You can try helmet silks. It will not solve your problem 100%. They do help. Buy a few and wash them regularly.
Cooling Skull Cap Helmet Liner for Men - Motorcycle, Cycling, Football Head Beanie & Hard Hat Liner - Sweat Wicking Skullcap https://www.amazon.com/dp/B071JH68QM/ref=cm_sw_r_cp_api_glt_fabc_JX2J774FF5ZVRWK8PF7Q
Any sort of hat under a helmet makes my head hotter too.
In the summer, I wear a skull cap like this one from Amazon
(No association, it was cheap and I wanted sun protection too)
I get it wet and sort of lightly wring it out before I leave and it keeps me cooler and catches any sweat in the cap. It dries out after a while but if I stop at a park or wherever to refill a water bottle, I wet it again.
a skull cap helmet liner, a backwards ball cap with a Bump Cap Insert or headbands.
Old and bald. I used this during the Texas summer and thought it worked really well: https://www.amazon.com/dp/B071JH68QM/ref=cm_sw_r_cp_tai_2ED5DbBQ6ZXE2
Get a helmet liner