It can be hard to differentiate mental vs. physical fatigue. They should both be respected, but I’ve found that they can be managed differently. It’s totally normal (and arguably desirable) to be running into fatigue as you approach a rest week and pushing through a little fatigue is part of the deal. But the workouts still need to be productive and they won’t be if you are getting too deep into a hole. Hard to know how deep you are right now, it’s really something you have to learn over time with trial and error. Personally, I look at the “macro” like quality of sleep and mood and I don’t put too much stock into how my legs are feeling. Legs are always going to be beat down a bit after a big block.
As others have said, the safe bet is to back off. But there are ways to test the limits a bit that are pretty low risk. When I’m feeling tired on an interval day, I’ll always start the workout. If I’m struggling to do anything after warming up, I’ll just make it a recovery spin. If I’m feeling OK, I’ll try the first interval and make a decision based on that. Sometimes you’ll surprise yourself. Sometimes I’ll get 45 minutes into a recovery spin and the legs come around and I’ll knock out the intervals and feel great. All that is assuming I have some motivation to train. If I’m burnt out on hard intervals and have months to go before my event, I just won’t do hard intervals and pick other ways to build training stress. I generally defer any intervals above sweet spot until ~3 months out from my event because I know I’ll get burnt out starting intervals 5 months out.