You were 17 when you lost everything. You lived in a small town with your family, when unexpectedly a bomb of some kind hit your town and killed your entire family and destroyed your town. You were the sole survivor of this bombing. You didn't know it then, but it was Hydra that launched the bomb as a 'test'. You thought you were going to died because of your injuries and waited for the sweet taste of death. But you somehow survived, and when Hydra went to see their 'work', seeing you alive still, sparked an interest, so they took you back with them. A doctor, known only to you as 'The Good Doctor', did multiple experiments on you, injected you with different serums. You find out he didn't want to do these experiments but was being forced to.
With all the serums being injected, you end up having abilities, over the next 3 years they kept you in a cell and forced you to "train". You would be sent on missions, but then brainwashed of it, every now and then when you weren't in cyro, memories would come back.
You made the decision to deal with all of this, but when you felt you were ready to escape, you would rain hell on all of them. So you trained, you gained strength, you gained agility in every way possible. But most importantly, you gained experience with your abilities.