Ok, everyone else has given you the 'chin up' & 'what you/we are doing is important' bits - they're good and important.
But here's what you need to think about when the complainers are getting to you:
1. Ask yourself first, if this is a person whom you respect.
If so, take the nugget of true criticism & say it back to her to make sure that you understand. If you are note-taker anyway, write it down. Then tell her that you will take some time to think about it and ask if she has any other feedback. Go home. Think, write, ponder and either take it or let it go - do not stew.
If it's not a person whom you respect, ask yourself this question: As I have such a low opinion of her, why should I give a flying fig about her opinion of me? When speaking with her, mentally picture an appropriate (mean) stand in for her - a sheep for the mom who doesn't want to change anything, a lollypop for the too-skinny one who keeps telling you how to do it but never shows up, a chihuahua for the yappy dad who always has to have a comment even if the topic has nothing to do with him and he knows nothing about it. Have fun with this part! Take criticism from these people very lightly & *never* personally. If they are leaders in the school, you might want to consider whether this is the right place for you.
2. Make sure that a couple of the ones you respect are with you on the Board. Make sure that they know you really want and need their feedback. If they don't tell you anything, call and ask for it - one on one and shortly after the meeting. Start the ball rolling if they don't seem to want to say anything. ("I think that I went too long on the review of the Carnival, but I wasn't sure how to move on. What did you think?")
3. Insist that criticism be given in way that has a chance of being productive. For example, another poster said that she always offers to meet with critics. You could also ask the critic to put her thoughts in writing - 'Jane, I want to make sure that I understand your thoughts on the budget so that I can address your concerns with the right person. Please drop me a quick note or email and I will get back to you within the week.' or "Debbie, you and I have very different communication styles and personalities. I know that you want this to be successful, but I'm not sure if we can work together on it and I'm not sure how much of your concern with the holiday program is personal and how much is about what works for the kids. Could you meet with Susie to work on that and then let me know what still needs my attention?"
Say any of these without anger, just very matter-of-factly. The subtext of course is "I'm a rational person invested in getting the best outcome for the organization and the school we support - - and if you disagree with my perfectly rational approach, you're not!"
Also, it makes the critics think through their issues and may actually result in feedback you can use - either personally or in directing the other parts of the PTO team.
4. Delegate AND insist on transparency in communication, status, financial matters.
The PTO Pres should not be the head of every committee and engaged in every activity. The committees should be reporting up about how projects are coming along (good & bad) and very open about financial status. The Pres is the leader and the one to hold others to account so that the group as a whole is healthy.
Make it clear that problems aren't the end of the world (usually!) and that airing them to the group let's more people help resolve them.
If you have a slacker committee, DO NOT DO THE WORK. Prod the team leader and then the rest of the committee; then remind them that the status of their project (whatever it is) will be on the Agenda for the next meeting in x# days. And follow through.
You will be called names for this approach - but not by the people who are actually doing their work. Only the slackers will have a problem with this approach and you don't need them anyway. As for the ones who decide not to like you, refer to rule 1.
5. Do not get weepy over this. Try to not lose sleep - Benedryl can help for grownups, too. Don't get too mad either, but a little dose of righteous bitchiness can be powerful.
Lastly, please remember that your role here is NOT to make everyone happy. If that were in the job description, none of us would sign on for it! Focus on leading the team so that you can support the kids and teachers. (And remember that it's only eight and a half more months!)