Don't be sorry
Something I have noticed in the open source community and also in myself specifically, is that people feel guilty for things they don't do. You haven't answered that issue from a week ago, just because you didn't feel like it? You forgot fixing that terrible bug in that library you started making for fun, but others started relying on? You don't feel like keeping to work on a certain thing anymore or heavily slow down the pace? Don't feel sorry! You are doing all of this in your free time. YOUR free time.
Users of software, whether it's end-user applications, libraries or whatever, will always have expectations. However, YOU don't have to meet these. You haven't signed a contract, youaren't being paid and you probably haven't promised anyone anything.
While I want to believe all of this myself, I find it to be very hard. However, feeling bad won't help me or anyone else for that matter.