recurrent early miscarriage

I’ve recently turned 38 I had A round of ivf/Icsi with immune drugs in December.

weve been trying for almost 3 years Ive had one definite miscarriage at 5.5 weekes about 20 months ago.Then I had on about 6 early positives only to start my period a day or two later. 

Ive had all the standard tests everything has been great.

the only thing is I have an under active thyroid that is treated.we had ivf which again resulted in a chemical pregnancy in December I was given drugs to suppress my immune system and it still didn’t work. I have no idea why pregnancy can’t last more than a couple of days.does anyone have any ideas on what tests I should get ? I’m so near to just giving up