I can only tell you of my own experience with this. We had a rescue cocker, and when he was about 8 I noticed a lump on the anus. Went to the vet, our usual one was on holiday and we saw a young locum. He said it might be cancerous, just to keep an eye on it. I didn’t,t check it every day, thinking it would be too often to notice any change, but in two weeks it was very much larger. Saw my usual vet, who took him straight in and removed a tumour, which sadly metatasised into his spine, so the outcome was very sad.
I'm sorry to hear you're worried you've been let down, but I would go with your new vet's opinion and hopefully get some thing done about it PDQ. Finger's crossed for you and Natti
Lesley and Dylan