@weeksy have you thought about risking managing your effort via the HRM less? Ride without it (or without pairing it to zwift, so you can’t see it during the race).
My problem (on the kind of course I enjoy, especially since I became a B) is staying with the lead group until the race gets interesting. So on, say this race last week which had 35km flat lead in to a climbing finish (https://www.zwiftpower.com/events.php?zid=1080840) I lost the lead group after 10km. After that it was a mix of lonely TT, shifting small groups, other cats coming through the field etc. Maddeningly, lots of riders quit the event ahead of me when they got dropped too. Admittedly that’s a stupidly long lead in (what was I thinking?) but a typical outcome for me on shorter course too. I’d like to get to the decisive climb on courses like that in a bigger group, but I am not (yet) managing it.
I’ve realised though that I’m managing the effort wrong. I get dropped partly because I’m trying to sustain a level which still leaves something in the tank for the finale. But actually I need to get as far as I can – even if it means I crack – with the lead group, sod the finale. Once I manage to get there, I can worry about that! I think – esp recently – you are riding in the same fashion and managing your effort to avoid cracking. My climb is like your final lap. But what’s the downside of risking it? Neither of us are getting as far as we’d like because we’re focussing on what happens immediately next. We get dropped not because we can’t hold the wheels, but because we judge (for whatever reason, rightly or wrongly) that we can’t sustainably hold the wheels.
I’m going to try one of the Box Hill Races at lunchtime (either the 3R one or the Chinese one with fewer riders but a shorter lead in) and set out only to stay with the lead pack as long as I can – even if it means sprinting well into the red at times – and stop saving anything at all for what comes next. It’s not a failure if I don’t PB the climb segment because I get there knackered, and I’m going to stop riding like it is.