I have the exact same problem!!!!!!! It really boils down to two things - too much stuff, and the sentimentality or I might need it someday issues. My parents died when I was in junior high - and I really think that led to my need to hold onto stuff. Some because it actually is from my childhood and therefore a part of my parents that I can hold on to, and some because I can have some control over what is mine and what isn't taken away from me. If that makes any sense...
Two things (both tv shows) that have REALLY helped me:
Clean Sweep!!!! I LOVE that show!! They are brilliant. Peter Walsh is just a genius. I use his system a lot. I tackle one area at a time (because my three kids are never gone for a week or a month or however long it would take me to do the whole house!) If you wait for enough time and energy to do a large space - well, I never have that much time!
So for example, I did our game closet. It is a closet in the upstairs hall. Top shelf is sheets; on the floor is tp and kleenex storage; all shelves in the middle are games (kid games on lower shelves and adult games on upper shelves). I went shelf by shelf. EVERYTHING on one shelf came out; I sat there and made two piles - one to keep and one to give away. You have to be ruthless. If it is a game that your family actually plays, and likes, and wants to play again - keep it. The others go. Yes, they are perfectly good games. Yes, Aunt Julie gave it to Junior for his birthday. But there is only so much room! If it is broken or missing pieces then it absolutely goes! Put back the keepers and the others go in a big box. Then do the next shelf the same way, repeating until the closet is done. Closet now is so wonderful to look at that you will smile for a week just about that. Box(es) of giveaway go on your front porch for pick up. In my area, you can call Services for the Blind and they will pick up for free. They then sort through and sell stuff I guess. There must be some upside to it for them. I do feel like we could have a garage sale and I could make some money back on the stuff, but we are too busy for that! Just get rid of it and feel better!
Ok, game closet done. Next day do the pantry. All the expired food gets thrown away. All the food that you just keep skipping over for whatever reason gets put in a bag to go to the food bank (we have drop off bins right in the grocery store - easy). Organize the canned goods, cereal, pasta, etc. I actually had bought Tupperware to organize pasta products (I hate it when you have a half open bag of penne or whatever....) but hadn't used anything but the spaghetti keeper. So the other Tupperware was littering up my kitchen cabinet! Ugh! But I dug it out and put the pastas in them and it is so much better!
One area at a time. Get rid of what you don't need. Take pictures of the kids holding their art project, then recycle the art!!! This isn't as hard after you do it a few times. You have the photo!! It is ok.
You need a place to put everything. I am great with cookbooks and kid art supplies and games because I know where they go. They have a place to live. It is mail and papers that the kids bring home from school and coupons and ads and lists that clutter up the place mostly. Our art supplies are great because I bought one of those plastic towers (white with clear drawers) and labeled each drawer with my label maker (love that thing). So drawers for: crayons, pens, paint/paint supplies, construction paper, coloring books, stickers, pencils and scissors. I know where they go. The kids know where they go. Babysitters or Gramma or whoever might be around know where they go. It is great.
I understand how hard it is to get rid of things. Really I do. But you'll feel so much better when you do!!!!
Oh, the other thing (I said two shows) was an Oprah show where two guys helped her go through her closet. Oprah! You can imagine how many clothes and shoes SHE had! But she said it changed her life to do it. She kept only things that fit three criteria:
1) Does it fit?
2) Is it flattering?
3) Is it the image I want to portray?
If it doesn't fall into ALL THREE categories, it has to go. They did give her three items that didn't. So maybe you keep your wedding dress even though it doesn't fit, and two other special items. But that is it.
If it is too big, please do not keep it and allow for the day you'll gain weight back! If it is too small, you'll deserve new current styles if you lose weight. Really, I've done this. You lose the weight and can fit back into those jeans but they are so out of style now and you just want (and deserve) something that is hip and fashionable after all the work to lose the weight.
If it fits but isn't flattering, why keep it???? You know how you feel so great when you know you look good? Feel like that every day! Wear only what is flattering to you. One of my three items is a paint/clam digging/whatever messy thing you do outfit. Old jeans and a crummy tee and sweatshirt. But just one outfit for that! You don't need four pairs of crummy jeans!!
Even if it fits and is flattering, is it the image you want to portray? Does it look like something a teenager or a gramma would wear? I got rid of my flannel shirt even though it fit and was a little flattering I guess. Because I want to look like a cool 30-something woman, not a cowboy.
Sorry so long. Having the same affliction myself, I have a lot of understanding around this issue!
I'm still working on it.