He Is Jensen, Part One by Lisa Renee Jones

He loved only one, but he was never what she needed, never the right man for her. The best thing he ever did was walk away from Layla before he destroyed her life. Proven over and over again, most recently when he ended up tricked by his own government into taking an “immunization” that turned him into something not quite human, a super soldier, a GTECH.
But as a threat develops that could destroy the world, he believes one beautiful, talented scientist might be able to help them find much-needed answers. That woman is Layla. The problem is that the enemy knows this, too, and he must get to her before they do. And once he finds her, will she hate him too much to accept his help and to offer hers? Because what neither of them can know, is that they might just mean life or death to each other.