Well, if a part of you wants to free up space but it's at war with other, perhaps there can be some sort of balancing act. If you have doubles, start there. Sell or donate the duplicates.
You don't have to get rid of everything in haste. Take it slow and easy if you wish. Out of 900 figures, even negating the doubles, there are bound to be some you don't have an emotional connection to, they've just been around forever and you've gotten used to them being there. So while you're in this state of "I have too much" go through and separate the figures - the ones you cherish from the ones you don't / the figures you only kinda-sorta like. Then you can think about selling or donating a few of them each month (ex: 1-5 figures). This way it's more of slow transition into getting rid of things rather then filling a box and throwing it on ebay as quickly as possible and regretting it later.
I find it helps when you have them in a place where you know you've made the decision they're to be sold. Sometimes you pull things out because you change your mind, but I find most stay in the bin and have their allotted fate. I will admit though it happens on a different scale for me. I don't have anywhere near as many Transformers as you. What I have are three FULL bins of miscellaneous toys from when I was a kid. Things I've hoarded in the sense that most haven't been played with in 10-15 years. So when my grandparents have yard sale, which usually happens once a year, I go through pick out two dozen or so things from those bins to bring out to sell. Last year I got rid of 8 Polly Pockets from the mid 90's. A little sad to see them go because I loved them as child, but knowing that other enthusiastic kids got them made me happy. Especially the one little girl who loved them so much she couldn't decide which one she wanted more (her aunt actually came back a short time later and secretly bought her the other one
). And you know, I've never truly regretted selling anything at any of the yard sales (or donating the left overs). It might be a little sad to see them go but it quickly turns into a shrug of the shoulders because I'd made the decision to sell them days ahead of time.
It's totally up to you what you do of course. This is just a method I use to get rid of things I find I'm only keeping because of nostalgia.