You are conflating an entirely innocent offer of being walked home with the notion of deserving punishment in the form of looking after the walker-home's children.
That is a daft notion & you need to rid yourself of it pronto.
Worse though, you have clearly also decided that your friend ALSO feels that because her DH walked you home once, you now owe her endless childcare.
She may think that - in which case she's not much of a friend - or she may not.
What you need to do is stop acting as if this is in any way true, you need to stop believing it yourself, & you need to stop assigning motivations to other people about their own feelings. You cannot control other people's perceptions, assumptions or feelings. You can only control your own, & - apologies if this is coming over as brutal! - you are beating yourself up over nothing at all, & accepting a 'punishment' which is not only undeserved, but likely all in your own head.
I can understand how as a recently bereaved widow your self esteem & sense of self in your community might have taken enough of a knocking that you felt that some mark had been overstepped by accepting the offer of being walked home. Believe me - it was not.
The good news is that your increasing sense of resentment shows that you are beginning to realise that you need to assert yourself, & that your feelings matter too.
Your friend is very unlikely to have positioned you like this due to any feelings on her part about your "owing" for the walk home (or the late night listening to music ffs). What is much more likely is that - in the way of favour-giving everywhere - no good deed goes unpunished & your friend has become used to leaning on 'good old Bobewing' & no longer has any clear appreciation of what a huge favour she is receiving. It has been going on so long that she now feels entitled to your unwavering assistance.
That's quite a lot for you to process & unpick, & I wonder how you feel about having some talking therapy to sort out where your self-esteem is currently at, why you have not felt able to be more assertive ... did you have any grief counselling after your DH died?
So there is the psychology aspect I feel you need to work on in order to generally feel more comfortable in your own social setting. In practical terms, dealing with your friend's rather pushy & entitled demands that you now step up & provide her with even more childcare, I'm going to RTFT as I'm sure you will have a lot of suggestions on how to manage your friend's expectations & assert your rights to not look after her kids any more, so will catch up on the thread first.
Please remember - you have every right to say 'no', every right to focus puely on your own children & yourself, & you don't 'owe' anyone anything.