I think I have been vulnerable to depression for most of my life. I don't mean that I have always been depressed, only that peoples brains and bodies react to things in different ways. For example, a possible side-effect to many anti-epileptics is depression but that does not mean that everyone taking them will suffer.
I have lacked self-belief since about 10 when I was bullied at school. Although, that only lasted a year a lot of the insults and feeling that there was something wrong with me, stayed. I also think it is partly genetic as I have members of my family who have struggled with low self-esteem.
I think it is quite possible that I would still have had some problems with my mental health but I believe that the medication exacerbated them.