I don’t think this is a problem Monzo can solve. As I see it, you could either go down one of two ways:
Accept that when you lend money you may never see it back again. As you’ve mentally written it off at that point, any you do get paid back to you can be seen as a bonus.
If you can’t afford to write off money you lend (or don’t want to write it off), then you need to always do so with a written contract. It doesn’t need to be overly complex, but should at least contain the most important bits of information - the parties involved, the sums involved, and any repayment conditions that should be met.
Broadly, something like “I, [name] agree to lend [name] [£sum]. This is to be repaid by [date] / At the rate of £5 a month / Repayments to be made monthly until cleared, or [date] at which point any remaining balance owing must be paid in full.”
Then use a simple spreadsheet - could even draw it up on paper rather than in a computer - to track repayments.
(IANAL, I’ve just watched daytime TV (Judge Rinder, specifically) when I’ve been home sick. And personally, on the rare occasions I’ve lent money myself, it has always been under the 1st circumstance.)