Ways to improve short LP

Hi luvlies, I think I way have a short Luteul Phase but have no idea how to calculate it. Any ideas? So I think the reason why I'm not falling pregnant is because I'm not producing enough progesterone in the 2ww to support pregnancy. I've heard Vitex works. Has it worked for u or can u suggest anything else over the counter that would improve my chances of conception before heading to the Dr office? TIA :c)