I guess I'll do a bit on this.
First way we can take this, negatively. The mother didn't really know how to handle Ri-chan's one-track mind to becoming a musician and couldn't really support her. I don't think she really saw a future for her and when Ri-chan did fail and fell into loathing and depression, she couldn't really do anything. She doesn't know how to support her now. She might have felt shackled down with her ex-husband and now her kid. Now that someone else is taking the burden of her, she's no longer stuck and can go back to step one; only this time, it might be too late for her. She's working a 9-5 with overtime often and barely has time to sleep or make food for herself. It's a lot easier though if you're not having to take care of a kid while doing this.
Second way we can take this, positively. The mother didn't really know how to handle Ri-chan's one-track mind to becoming a musician and couldn't really support her. Her life is so busy that she was having to dedicate herself more and more to work, especially after divorce. She knew that Ri-chan's life would need to go somewhere towards an actual career rather than music. But, she didn't have the heart to force her to change, she knew that Ri-chan retreated into music and Tatsumi's company in order to get away from her parents' divorce and couldn't break the final thing which was holding her together. After she failed and fell into depression, she couldn't do anything due to her hands being tied. But now with Tatsumi willing to take her to Tokyo, Ri-chan now has the support that she couldn't give her. She's unable to say sorry for her shortcomings as a mother, but hopes that whatever comes next is better than whatever she was able to provide for Ri-chan.
Ok, done cooking.