So sorry to hear you're going through this.

A few years ago I had an ultrasound on the lymph node in my neck as I had a hard, painless lump there. Turned out it wasn't a single lump but a few very small nodules. 
I was told that people can get growths like this and it can be completely harmless but just to keep an eye on it in case they grow. 
 

Three years on, I feel the lump has grown and I'm waiting for a GP appointment to get it checked out again. I'm especially worried because I also developed a non-itchy rash on my chest that lasted for months and has now formed into a small bump.

I think your concern is completely valid - no doctor can tell if a lump is or isn't cancerous just by feeling it. But at the same time, it's important to remember that most lumps are benign. 

Keep an eye on if it grows and any other symptoms like weight loss, night sweats, rashes and fever (even low grade) and don't be afraid to go back and ask as many questions as you want. You could also try requesting a biopsy and see what they say. That's the only sure way to tell.

I do understand your worry but if you're feeling well otherwise, I think that's a good sign. And if you know you're keeping an eye on it and the odds are in your favour then I hope that will help to ease your mind a little bit. 

Wishing you all the best!