Hi Steven Eleven,
You definatley aren't a dumbo. I know a lot of guys who seem to stay with the same girl for a while.
If you have fun, you treat each other nice (no arguing or jealousy), and she is not trying to extract all your hard earned out of you, then there is nothing wrong with the same one.
The problems arise when all the silly stories come from the girls, and you know they are just taking the piss. Once you see these signs, then it should be time to trade in for another 'nearly new low mileage'
bar girl.
A lot of guys butterfly so they dont get to that stage. It all comes down to what is best for you. If you are the one in control, then it should always be great fun.
Adios