I can, if you so desire, use demonology, evoke the power of a demon and have her have an "unfortunate accident". Or if you prefer, I can prepare a nice little talisman which will force him back to you against his will.
Are these things you really want? Make a deal with a demon, do things to make him go against his will, have her harmed or killed?
There is a price for magic, and dark magic - black magic as you would call it - carries hefty prices.
Now what I learned as a person residing on this planet for a long, long time...
Everything in life has a beginning and an end.
Everything in life contains lessons, and whilst those lessons may hurt like nothing else we have experienced before, we usually come out better for that experience.
For every loss there is a gain. While you may not see this potential gain, there is one.
No you are not saving your marriage, you are trying to save the memory of what your marriage once was. Understand the love cooled round year 2 and he has been moving away from you for 3 years. This is just the physical manifestation of his growing cold.
Let him go. He cannot give you what you want - and definitely cannot give you what you need.