Why did my mango pickle become bitter?
My first thought would be you over-roasted the methi.

Fenugreek seeds benefit from a light roasting or bhuna which removes the slight bitter edge they have - however, if you over-do it that bitterness comes back with a vengeance.

To avoid the issue, I usually add fenugreek powder to my sauce as it's cooking. It still develops the full aroma that way. It adds a very slight bitterness, but removes the danger of over-roasting right at the start &amp; not finding out til right at the end. 

I'm not sure it can be 'fixed' as such, other than by trying to balance [or really mask] it with sweetness, more acidity, or trying to dilute it in a larger batch.