Hey Sig, I don't think you did anything wrong, I don't think you're an "idiot" at all, and I don't think you're chasing him. I think you're a MOM. You love your son, You let him know how important he is to you, you texted back your truth about the situation, and he didn't respond. I think the only way for you to feel better about that is to let go of your expectation of how you want it to turn out and accept the way it did turnout. I think you did a loving thing, a gesture which he got, believe me, but when you're 20 years old, even if you're a easy child, as a friend of mine told me, "they're terminally cool" and act out of that coolness. My guess is his lack of response had nothing to do with you, he got it, thought, cool Mom, and went on with his day. They are just self centered, even in the best of circumstances.
For me, the way to not feel that big disappointment is to let go of my expectations about the way I think it should be. Perhaps because I am SO OLD, I have fallen into the acceptance part of life a tad easier, been at it long enough that's for sure. But, it's true, to the degree I have expectations, is the exact degree of the size of the disappointment. There is an Native American process called the Four Fold Way which is a conflict resolution process which is ......*show up, *pay attention, *tell the truth and *let go of the outcome. When I am able to do those 4 things, for me the letting go of the outcome being the most challenging, I can usually handle the conflict well. In any case, it's a good tool.
I believe you are simply doing what any loving Mom would do on a child's birthday. The snag was expecting him to do what you thought he should do. If you let go of that, you can feel good about his saying, thanks Mom. From my outsiders view, he got your love and your birthday wishes. Feel good about that and let go of the rest.