This is probably a temporary problem, in response to a 25% increase in mileage. Though 15 more miles per week does not sound like a lot, it's a relatively large increase for someone moving up from 45 miles per week. 45 a week is less than 7 per day, so adding 15 miles is similar to squeezing in two extra days of training each week. Given that, it is not surprising that your body is taking some time to adjust to the greater workload. But, that's what training is all about. When your racing hits a plateau, you have to increase the training load to make further improvements. Once you're at a fairly high level (as your training load suggests you are), it gets harder to gain improvements.
So, I suggest:
1. Long term, stay the course. Maintain your self confidence. If you were good at 45 mpw, you'll ultimately be even better at 60.
2. Short term, take one week at your old mileage of 45 to freshen back up.
3. When you are increasing training load, do it by adding miles OR intensity, but not both at the same time. ALternate the manner in which you increase your efforts.
Good luck, and kick some ass at the end of the season. You're training hard