Futuremature - I can relate to everything you say, and also to the hurt in your post about how it feels to be treated this way.
I can only speak personally, and I'm not much older than you (36), so please take what I say with a pinch of salt! And apologies, because this is long! What I want to say is: I don't think solving this is about noticing toxic 'signs'. I think it's about you relating to the world in a different and new way, which will draw the right kind of friends to you. The reason this is long is that, if I just left it there, it might sound like I was blaming you for the selfishness of other people. I'm absolutely not - but I'm saying that in my experience, if you want to escape these people, you have to send different signals.
I realised at about the same age as you that I had slipped into a habit of feeling like I wasn't worthy of space on this earth, as a result of problematic family relationships when I was growing up. That sounds really dramatic - but it wasn't actually a big histrionic thing at all! I simply approached every relationship with a view to 'What can I do for this person?' because my immediate assumption was that I wasn't worth knowing in my own right, and had to give, give, give in order to be someone's friend.
The result was inevitable: whenever I left the house, it was as if anyone with any problem could smell the pheromones. I had random people sobbing to me at the bus stop, people stopping me in shops and telling me deeply personal things about their lives, cashiers crying in the supermarket... you name it. I couldn't go to a party without ending up in the loo comforting some stranger who was alternatively being sick and crying! Inevitably, my 'friends' were all people with significantly problematic attitudes, who believed my own evaluation of myself, and lent on me without mercy or reciprocation - financially and emotionally.
Bizarrely, the key to this ending was when I met someone so utterly and unreflectively selfish as a friend that their behaviour made me stop and wonder why on earth I was doing this. At around the same time, I read that Bradshaw book on The Family - and while I thought it was a bit simplistic at times, it did describe patterns of behaviour that I recognised in myself. I came to see that I was very much inviting all of this behaviour into my life, and that I'd never have a good social existence if I didn't do something about it.
I didn't 'bin' the difficult friends, but I did start to set limits. I stopped lending money to people who would never pay it back. I stopped being available 24/7, e.g. answering the phone at 2am to friends who were drunk and teary. When people abused me, I quietly explained that they had hurt my feelings. I still did all the big stuff (hand holding through dark times etc.), but I refused to be 'on' 24/7. As soon as I started to do this, the people who were really using me fell away of their own accord, because I wasn't giving them the unending attention and patience that they wanted. Some friends, huh? As soon as I stopped giving unconditionally and excessively, they dumped me! Fortunately, at the same time, a new bunch of friends appeared in my life - as if by magic! I realised that it was simply that the effort that these toxic relationships were costing me had actually got in the way of making better and more healthy friendships.
The other thing is something about closeness. When I was growing up, I had a circle of extremely close friends, and very deep trust and connections with them. I've come to realise that this tends to be a wonderful, but temporary feature of youth! (Not to say that there aren't close relationships in later life, but they are rare and precious). I now have many more friends, but at a less soul-to-soul kind of level. I don't expect them to be perfect or on call all the time, and they don't expect that of me either. It's nice.