Last yr I paid 50K capital gains tax. This yr I've 200K unrealized capital loss. If I realize some of this cap loss can I get some of last yr's paid cap gains tax back?

Short answer ... NO. What happens in 2022 cannot affect 2021. 

Any  losses realized in 2022, and not used in 2022, will automatically carry forward to 2023 and beyond.  But, 2022 losses cannot be carried back.

@hemandave80 unfortunately, it is a one way street:

As noted, if you had a big gain last year and a big loss this year, you can NOT use this year's loss to offset last year's gain

HOWEVER, let's say it was the other way around:

if you had a big NET LOSS last year (which creates a loss carry forward), and had a BIG gain this year, you could carry the loss FORWARD to offset the gain.

 but you can't carry the loss BACKWARDS to offset the gain, hence a 'one way street'

If you did this in Futures (Sec 1256) then you could carry back losses.

This treatment is not available for stocks and bonds