The simplest answer is in fact your own: _only_ being able to sign sign our own trade deals, for which no other country is rushing to negotiate. The USA has made it clear that they want to degrade UK agricultural standards to put US farm goods (lower standards and mass monoculture) on an equal footing with British farmers. But doing that would harm trade with the UK's largest trading partner. They've made it clear that they want the NHS to pay more for US pharmaceuticals.
Moving on, as you've obviously not been paying attention, the Irish situation is dire: either we have a hard border between NI and the South, or we have a hard border between two parts of the UK.
Another great problem is the fact that London has ceased to be the financial capital of Europe. The effects of this are being masked by COVID, but they're real. Similarly, as of January 1st, certifications for goods issued by the UK cease to be valid in the EU, which is going to cost British businesses over their European counterparts.
I agree that all of these are, individually, manageable. But turning the Brexiteer's tub thumping around: what benefit -- in pounds and pence -- is Britain gaining from having left? Yeah, Westminster loves being immune from having to follow rules they didn't (exclusively) write, but is that good for Britons, or good for politicians?
One factual observation: Brexiteers claim that Britain will not be subject to foreign courts, but just a lie: if we want to trade, foreign courts will be part and parcel of any deal, including (obviously) the WTO tribunals. Also EHCR (hopefully; xenophobes hate it, because it sets a standard for human rights that some petty tyrants can't disregard). ITLOS, and so on.
Lastly, the claim that "we're never going back" is suspect and inherently naive: Northern Ireland is the closest to "going back", but the Channel Islands and Scotland both recognize a different balance of probabilities...
Biting the hand that feeds IT
