S.P.
It could be any number of things! You might depressed, you might be tired, you might need a friend, or it could be medical! I would have a doctor check your thyroid and B12 levels. I would also try exercising as often as you can. Maybe join a mom's group like stroller strides, so you can take the little ones, or join a gym that has childcare (most YMCA's do).
As far as your "friends" go, it sounds like you need new ones. Join clubs for yourself to try to meet new people in similar situations as you. You will be able to develop friendships because you have things in common.
I guess you can't really get rid of your in-laws, but I would just stop trying to depend on them and find a good babysitter in your area. Also maybe don't be so available for them. Do what you can, but don't go out of your way to do anything unnecessary.
I know it is hard but you absolutely have to take time for yourself! Maybe sit down with your husband and explain to him how you feel and agree that for the hour after he gets home from work he is on duty so you can do anything and everything you want from getting your haircut to taking a nap or just going for a drive with no kids (and kids songs) in the car.
I have been depressed and I refused medication. There are other options if you don't want to be medicated! Exercise helps because it helps develop a positive image of yourself which will translate to the rest of your life.
Also, I went to hypnotherapy. Hypnosis kind of has a bad rap because it is used for comedy, but it did wonders for me and completely changed my outlook on life. Please feel free to message me, and I would be happy to explain my story and how it helped.
Stay strong momma, you will get through this!