Do I have the chance to get my son back?

am the mother of an amazing 12 year old , when he was 8 I went thru a terrible rough patch and wanted what was best for my son,the only place I could find work was as a civilan contractor in Afghanistan and a then family member offered to look after my son, I signed temporary custody papers while out of the country , She promised me it all , she would never stop my son from seeing me , never stop phone calls , emails and skype etc….. She THEN told me if I didn’t sign adoption papers she would go to the courts and say I had abandoned my son …….. My son and I had the most amazing relationship ….. I signed the papers ( I made a bad choice) she then in turn once she got what she wanted stopped all calls, skype , visits ,cards etc….., My head was spinning thinking my son was expecting these things and I could do nothing about it she would put the cards etc.. in the trash, she would often send me texts saying my son thought I was just a cash cow etc……… My heart is broken and I know inside my son is thinking my mum just left me. I was going to try and visitations thru the courts and try to explain to a judge the stress this woman put me under.. Does anyone think I have a chance?