This would normally be a sign that you need to work more on your strength in the off-season/winter base and reduce focus on increasing stamina. So, from February through to April, you’ll ideally then have additional muscle lactate-clearing ability so the legs are strong and fit.
The critical decision for most athletes is to decide when you need to focus on supplementary strength sessions (for instance, gym work), bike/run strength sessions (perhaps hill strides or low cadence inclines), aerobic stamina work or recovery.
Look at the sessions you do and try to see if there’s a missing component. Do you include low heart rate skill sessions as well as harder workouts? Do you focus on stamina but not strength work (such as low cadence hills on the bike)? Are you recovering every three weeks by reducing volume and effort? And are you eating and drinking enough during and after sessions to ensure recovery? The devil, as ever, is in the detail.