Question about "abnormal ECG for my age"? -normal variation?

Hi everyone,
I'm new, and appreciate anyone who takes the time to check out my question. I realise it's pretty long!

Basically I've suffered with palpitations on and off since I was 16 (I'm now 25) and it's always been linked with my anxiety disorder (which I developed at 16) I've been up and down with panic/anxiety over the years, and about 3/4 years ago I got sent for ECG, which led to having 24hr holter and echocardiogram tests. Back then, apparently nothing was wrong as I didn't need to go back for any more consultations.

Rolling around to about a year ago, I was now in a different town so the hospital and cardiologist was different. I had an ECG taken that 'didn't look normal' to the cardiologist. He put me in for another echo and 24hr, I then saw a new cardiologist a year later (as I kept having to put off the consultation) and he said there were palpitations (sinus tachycardia) but said it was nothing unusual or worrying. He then said he wanted to put me in for a treadmill ECG and an event recorder. I have just completed these tests (did the stress test yesterday) and whilst I was waiting to go on the treadmill the nurse said she could see the abnormality on it, and said I have an unusual tracing for my age - which she said could be a 'normal variation'.
Anyway I have been having another spell of lethargy and feeling unwell (which I have always linked to anxiety and being run--down) so I only managed 3 minutes on the treadmill!! I am not overweight, but I have felt physically unable to do much exercise over the last month. The doctor said I've had a virus but anyway, sorry I am straying from the topic.
Obviously all this talk of abnormalities alongside my feeling ill isn't doing my anxiety a whole bunch of good! So I'd be so grateful if anyone can enlighten me on what this 'abnormal for my age' thing could mean? Does it mean I my heart tracing is like an old person? And doesn't that mean it may not be right?!

Also any clues as to why I was never told my ECG was abnormal before? Could it have changed in that time?
I know I will be seeing the cardiologist to discuss results but waiting isn't easy!

Thank you.