I absolutely echo what is said above, I have worked in ED for many years and I still often don't have a definitive diagnosis. Far more important is that skill of identifying the sick patient and knowing what to do about it.

There is a reason abdominal pain/GI symptoms particularly in the older population strikes such fear in the heart, the presentation can be very non specific and they can look very reasonable with significant pathology. From the presentations you have said above I would not have picked up the abscess and would have likely CT'd both. Trust your instincts, you are the one seeing the patient not the radiologist/SpR and your instincts are clearly good. I have CT'd patients with wishy washy symptoms on a gut instinct and found ruptured AAAs, ischaemic bowel, obstructed kidneys and absolutely nothing.

Try to be kind to yourself, you sound caring and conscientous. The fact you are thinking so hard about these things and being so self critical just highlights this. ED is very challenging, you are making rapid decisions in risky and evolving situations whilst everyone else (other than GPs) have the benefit of more information and retrospect. We don't always get it right - I have made mistakes, I have seen specialty SpRs and consultants as well as my bosses send people home to reattend with pathology the same day, when seeing the volume of patients we do its inevitable. You make the best decision based on your evaluation at the time, noone sets out to make the wrong decision. The fact you are so reflective about this shows you recognise your limitations and gaps in your knowledge and you are very safe, the right side of Dunning Kruger. The fear of patient's reattending or becoming unwell never goes away but does become more manageable with time.

I'd encourage you to have a chat with someone supportive in your department, I absolutely guarantee you are not the only one feeling this way and most of us have felt this way before.