Problematique

Hey fellow tb'ers. I couldnt think where to put this post so I decided here is the only place it could fit into...

I love playing bass, I have just had my 19th birtday last week and I have been playing bass for about 6 years now on and off. I am quite small and slender for my age, about 5ft 6 and probably about 120 pounds.

Now the problem that I have is that I cant play standing up, well i can "physically" but my playing gets alot worse from sitting, the bass digs in and I seem to tense alot and I get tired quicker than I should. I have tried countless straps and at the moment I have quite a thick one, but it still digs and my shoulder muscles ache...

My bass is a fender urge 1st model (not 2nd) so its quite small and light compared to a musicman or something...

Is this something I will have to get used too? Or like have I got to learn to relax more I dunno, I would like to hear your thoughts on the matter.

Thanks!