[susan53]

"since" means "from a specified point in the past up to another specified point"- ie the time fram covers two specified points. So its meaning precludes it being used with any verb which specifies one specified point only - eg present/past simple. it can only be used with perfect verbs which express a time frame encompassing two points. Eg :

I have lived here since 2002 (time point 1 2002, time point 2 = now)
In 2008, I had already lived there 6 years. (time point 1 = 2002, time point 2 = 2008)

When the present perfect is used, the second time point is always "now" and doesn't need to be overtly stated. When the past perfect is used, both time points are stated or retrievable from the context.

So, in your example, neither am nor was is possible, as they each express one time point only ( am = now, was =a past point) . Only have been can be used, as it combines both past and present.