It's easy to forget that an existing buff, aura, control spell is using concentration, especially ones you cast once per long rest.
It would be nice if the UI would add a hard-to-miss alert when you are about to cast a new spell that will end your existing concentration spell.
For example Shadowheart has cast Bless then the next turn you go to cast Sprit Guardian, Shield of Faith, Beacon of Hope, Bane, Hold Person, Silence, etc. -- oops, you've just lost Bless!
If when you selected / moused over a spell the popup said Warning: ends existing spell or possibly better if after you selected it you had a warning and chance to cancel it would save some grief.
Even after I learned about concentration I've accidentally ended bless, haste, and others a few times.
Even better would be a feat (or addition to war caster feat?) allowing you to concentrate on 2+ spells at once.