Why do you feel guilty? It's Mothers' Day. You have no kids together, her kids are grown-ups who can prepare gifts, she left you, she has a new boyfriend and she doesn't want you to pursue. She does not want your gift, or involvement, and will not be pleased by it. It seems a very clear case to me. Is it really guilt that you're feeling or something else?
M39 D6 D3 (at S) S 2014-09 D 2016-09
"You can't start a fire sitting around, crying over a broken heart" - Bruce Springsteen.