Feeling Like a Horrible Mother....

I've just been sitting here at home watching my son play and I've been deep in thought. Depressed really. I worry I'm not providing my son enough. I struggle like crazy with money, and I barely make enough to make ends meet. There's not usually money left for "extras." As I watch my son play inside with his toys yet again, I feel so guilty. I never take him out to enjoy new experiences. I mean sure we go to the park or other "free outings", but I don't take him to, say the zoo, because I can't afford it. I feel like my son is missing out because mommy has no money. And I just feel awful. He's only 2 right now, but I feel like he knows somehow. Well, at the very least I can tell he's bored with toys and the park. How can I give him more when the budget is stretched so thin as it is? Will he look back on his limited childhood as boring and inadequate? Am I inadequate?

Edit: thank you all! You lifted my spirits. I need to get out of my own head sometimes.