Venlaxafine has contraindications for older people so might be a good idea to discuss that with your doctor,
It sounds to me that you are too poorly to do any of the things people suggest, like going for a walk, so don't beat yourself up about what you can't do just keep on keeping on in the short term but I urge you to go back to the doc sooner rather than later.
MoodGym is supposed to be a good thing to help but, like all these things, some like it and some don't.
I think it is hard for a GP to assess how ill a person with depression is so it is often necessary to go back several times before they realise the severity of the situation. Perhaps you could explain how you feel, do you cry a lot? Are you beyond crying? Would winning the lottery make you feel better? Tell him/her what the issues are which you are unable to cope with and let him/her decide what you need to help you deal with it.
I wish you well and encourage you to pour out your heart on here, it won't do any harm and might just help a bit.