To be clear... you are 315 lbs? Yes, you are too big for a TTR125. How tall are you? As far as a "full size" 125cc or 250cc (4-stroke) motocross bikes; they are the generally the same height a a 450 ... so depending on your height that will be a much better fit than the small TTR, not to mention a LOT more power than a TTR, which is a trail / back yard bike.
Most new MX bikes are setup from the factory for the "average" size rider, somewhere in the 150 - 170lb range if I recall, so mater what bike you get, it's going to be under-sprung for a bigger rider. You can still ride it and have a lot of fun as a beginner, it will probably feel pretty fast ... but keep in mind it won't handle as well as it should and it sure won't deal with hard landings... a sure way to get bounced-off big time.