For mental side, you just need get accustomed to this specific effort. But can you describe physical difficulty more precisely to help you find the root cause, something like:
- tiring quickly: lacking muscular endurance – Tempo/Sweetspot is good way to improve it
- burning feeling in quads: you might be near threshold but slightly above and it is accumulating faster than you can consume it – either focus on bigger base with Z2 to improve lactate consumption machinery or do over-unders to improve lactate shuttling and tolerating it
- breathing gets too hard, can’t suck enough oxygen – raise ceiling with VO2max
I am not a coach, some recommendation might be off but point is about describing feelings more precisely.