I learnt somewhere that 'mo' (and 'do' etc) in Scottish Gaelic are for things actually attached to your body and close personal relationships (ears, eyes, girlfirends, parents etc - e.g. mo phàrantan) and you use aig (agam etc) for everything else e.g. "an càr agam" (my car etc). Very interestingly you say "mo bhean" (my wife) but "an duine agam" (my husband - 'the man at me') - glè neònach (unless that says more than we care to discuss about mariatal relationships in the Gàidhealtachd)! I believe this is slightly different to Irish where mo and do have replaced the 'agam' style construction... but I don't know Irish so don't quote me on that.
Disclaimer: I am just another learner and not a native speaker.
I believe that it is because the construction for possession "the bedroom at me" means 'my bedroom' because the definite article here is bound into it in the same way that an seòmar_cadail seo would be translated as this bedroom and not the bedroom here (which would be an seòmar-cadail an seo - I think).
To say I like the bedroom I have, I think you would have to say Is toil leam an seòmar-cadail a th' agam literally the bedroom that is at me. In English, the word that can be omitted but is still implicit.
I stress again that I am also a learner so hopefully a mod or native speaker will wade in!