Hi mdb, how come you never see the same doctor? you should be able to do that if you ask specifically for an appointment with that one. I think it's essential you have continuity in the doctor you are seeing and that there is some trust and understanding between you. I don't know hat the circumstances are but I would always tell others to keep looking until you find a decent GP, you don;t need to provide any reasons for asking to see a particular doctor or for moving to a different surgery.
I think it is important to be honest about your drug use when you are receiving treatment for mental health, as it's a contributory factor as you have recognised yourself and it seems only right that you doctor should have the full picture. But I understand your reluctance if you have no regular doctor and are effectively meeting a stranger everytime you go.
You don't say what meds you are on but of they are straight ADs rather than something like Benzos that you are using then telling them about your drug use should not make you look like a drug seeker. That said I'd leave it to the doctor to suggest the best course of action, it may be a dose increase is required or they may suggest and change or additional medication. I'm not suggesting you just have to accept everything they say, it should be a 2 way conversation and you should express any concerns you have, if your not happy taking something then say so, IME this is where your honesty should give you some credibility.
There can be downsides to admitting to such things, they will be more cautious about prescribing anything habit forming like benzos, but then who gets a long term benzo script these days ( yeah I know you got one Brimz

) and I'm sure you can get those if you want anyhow. People will have had there bad experiences with this approach, as I have but the vast majority of the time it has far more positives than negatives, even recently when I had an accident and went into to hospital via an ambulance and had to have an op, I spilled my benzo use straight away as I thought they might need to know for the general and as I was staying in. I still needed to take them and really couldn't face trying to hide tablets and take them in my condition.
My partner brought in all my medication and the staff nurse was fully aware of the situation with my Diaz and was fine with it, even reminding me to take them specifically. I think a bit of honesty with doctors can be a breath of fresh air at times, I;d only tell them in situations where the information is relevant and in this situation it is.
Good Luck