As someone else has mentioned, it may be partly impostor syndrome. At least for me it is. But being out for so long (> 4 years), I can slowly accept my womanhood independently from how I present or how people address me. That doesn't mean I'm unaffected by transphobic comments or misgendering (which I still frequently get), but that I can get angry about it instead of feeling like an impostor. I'm more and more in a position where I can claim my womanhood despite people denying it and that is a big win. I'm also less prone to feeling like shit when I haven't shaved in a few days etc.

Regarding your questions: I live in Germany and in my city there are no lesbian/women only places that I know of, but most are open to women, lesbians, inter, non-binary, trans and agender people (wlinta or FLINTA in German). So these are places that should be a safe place for me as well. It still took me a veeeery long time to dare to even enter any of these spaces because feeling like an impostor. And in my case it was definitely more of my own fear then people actually excluding me.

Although I hardly know any cis lesbians and the ones I got to know were friendly but clearly not up to dating trans/nb people. As you said yourself, the available dating pool is frustratingly small. I've also had various frustrating experiences with heteroflexible cis women (and even cis men). I guess mostly cis people are just very confused about me and their emotions towards me and in the end it always gets messy and I feel like shit because I feel like it would be so much easier if I were just a cis woman... :/

T4T is great and all, but we are just so few people and it's hard to find anyone...

Transphobia in lesbian spaces is there I'm afraid, I feel like it is somewhat there, even if in a ""non-malignant"" kind of way, in any space with cis people just by virtue of societal values and a long list of etceteras. Best advice I can give, if possible find a space that is explicitly trans-inclusive, not just LGBTQ, but Trans in particular, you'll have more of a chance of lesbians there being more open and experienced being around trans people, not a silver bullet by any means, but it'll be a bit better.

So yeah, trust me can relate, we are all together climbing this mountain, just make sure you stop from time to time to rest at some safe spaces, we'll provide some needed break and some virtual hugs and support, best of luck to you in the climb sister!