I think there are a few reasons. As you say mentality, changing team due to injuries, less experienced  players and probably confidence. If you are losing a lot of games even if playing well it can affect confidence, particularly if you are not scoring many.

Hopefully  a current or new striker will start scoring a few, the team will put a few wins together and confidence will improve.

I think it is mentality with a touch of fitness. We begin to panic when conceding the first goal. We also struggle to hold on to victories, or keep teams from scoring in the last 90+ minutes. Rochdale home and Oxford away come to mind. This part might come down to fitness, but, at least previously, when we were down it we would score. We would fight to score. It seems that fight is just a frantic mess, or we just breakdown, and loose the fight. I think the team overall is made of stronger stuff, but it hasn't been showing as of late. 

I feel like the end of the 22/23 season came at the wrong time for us, when we had just started to work out reliably effective patterns of play that gave us a good string of results to keep us up at the end of the season. After the summer break we appear to either have forgotten a few elements of what was working for us or struggled to adapt to new challenges in 23/24, an example being going more direct in the second half of 22/23, whereas this season it feels like a pass back to Harrison is the only play certain players have in the locker.

We definitely have the quality there, no one who saw the second half of the Oldham game could deny that, just need a bit of luck to go our way and a few more goals to get the confidence up. I've also got to say the majority of defeats I've seen have been down to one-off individual mistakes, rather than the team being overwhelmingly outplayed by the opposition.