First off, are you an introvert or extrovert? Perhaps you are feeling lonely because you need lots of time around people to fill you up. You can't
I second the suggestion to join a gym, it will do wonders for your mood and confidence, it has for me. If you stay consistent and take classes you have a chance of clicking with someone.
It takes time to build new friendships and I get the feeling it gets harder the older we get. People get sucked into their family lives and any spare time they spend with friends they have likely had for years, leaving little time to work in another friend into their life.
I only have two kids but my youngest just started preschool so I get a taste now of what it will be like when they are both gone 7 hours. I feel like I should be lonely but I'm not because I really enjoy being my own best friend.
Take this time to connect with yourself and enjoy this period in your life. It can be lonely, but it will be more lonely in the future, at least I think so. Once the kids grow you will not be able to justify remaining a stay at home mom/wife. If you really want to work after your vacation go for it! Being around people and keeping yourself busy with work tasks might make you feel less lonely.
Think about the things you did pre kids that made you happy, start doing them again. Go to the library, find some books to read. If it's money you want, try selling random items on ebay, you get 50 free listings a month and it's kind of fun.
Seriously though, start working out if you don't already do so. It's taken me a year to get into a groove but I feel sooo much better than I did a year ago when I still had ppd! If anything, heading out for a walk and seeing other people walking/jogging/biking, ALONE, will make you feel less lonely.