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... its time to "give up" guitar when it doesn't give you pleasure any more. Clearly, that day has not come.
And if the rule is "if you're not good at it, don't do it" most of us wouldn't get off the couch.
Obviously time is an issue. If you want to keep playing guitar - and as Cary mentioned your letter just screams that you should keep playing - you need to cut back on something else. First thing you should show this letter to your husband. Make it clear that this is important to you and his little comments aren't appreciated, and neither are the kids'. See if you and he between you can't carve out some time from your schedule. What's he doing while you're taking the kids to their lessons? Any way the kids can get there themselves? (I don't know how old your kids are but I walked, biked or took the bus to swimming lessons starting at 9.)
Your whole life seems to be about work and your family. Where the heck is your time??