Alex I can totally relate to your original post as I was in exactly the same position as you 4 months ago, here's my story...
My other half has ridden bikes for years and after a break he's just got back into it and I was curious...
Having never ridden a bike before I booked in to do my CBT on the Saturday, Mod 1 training on the Sunday and Mod 1 Test on the Monday.
CBT was ok on the 125 although I was nervous.
Mod 1 training day was horrendous, pooped my pants completely on the 600.
Mod 1 Test day, Mark (Mr Able) dropped me down from the 600 to the 500 but I still pooped myself - I couldnt even keep up with him and the other pupil on the way to Bristol as I was so scared I was holding the traffic up and taking the corners at 20mph (I kid you not!).
At McDonalds coffee stop (Mandatory on any ride out with Mark!) I completely lost it and chickened out of doing the test - I was even petrified knowing that I had to ride the bike back to Trowbridge!
I was so cross with myself for chickening out as Im not normally a quitter, but I knew that I wasnt safe on a bike, not for my own safety, or for other road users.
So the next morning I popped down to Phoenix and bought a 125. At first I was terrified but I persevered. It helped that my partner accompanied me on his bike which really helped with my confidence. I rode that little bike at every opportunity and in 4 weeks I clocked up 2500 miles. At the the end of 4 weeks I was throwing that little 125 around like an old hand, cornering on rails (ok not quite, slight exaggeration but you get my point ;D)... I was totally in love with biking and I wanted more.
So, I rebooked a days training with Mark and went off and did my mod 1 on my own bike and passed. It helped that I took my own bike as I was comfortable and familiar with it. Yes it means Im restricted, but for me personally, it was the only way I was going to do it.
I booked up for my mod 2 a week later and ordered my new rides ;D
Passed my mod 2 and came home to my new bikes...
I was really sad to see the 125 go, it had built my confidence and given me valuable experience, but it was time to move on and up.
I now ride a Kawasaki ER6f to Bristol every day (for work) and a Harley Sportster XL 1200 custom for lols.
Both bikes are restricted, but I cant tell the difference as Ive never ridden them unrestricted
I absolutely love them to bits and Im still gaining experience and confidence all the time. I certainly have never looked back.
What worked for me may not necessarily work for everyone as we are all different. Which ever path you choose, good luck and have fun. If you want to talk to me more feel free to PM me. Im happy to meet up and ride out with you if you do decide to go down the 125 route as it helps to have someone with you I feel.
Biking isn't for everyone, but don't give up until you know you are sure.
Ps, just read this back and didn't realise it was such a long one, sorry! :