The other answers explained what you can do to mitigate your losses. I would just like to add that you, like many before you, need to adjust your mentality when on the market. The people you interact with on the market may seem friendly, now, some of them may even seem the friendliest on Earth because this is part of their job and they may even have dedicated resources and research behind this effect.
What you need is some pretend-egoism and healthy level of paranoia. In the market, you should never give anyone for free and undocumented anything that they ask for. If you come up with some kind of plan that warrants for whatever reason this kind of behavior on your side, it's your idea and risk, go with it and learn, it may even work out as intended. But many people are trying to put the pressure on you to take it from them. One thing market is, is a game. It may be not good to be completely devoid of empathy there, so I am not suggesting that you make use of people that let you use them, but you definitely need to be less trusting and assume others are making use of you.
It would be very hard to bring justice to everyone in your position without sacrificing other values that market economy provides, so the responsibility is mostly yours to protect your interest. And the way to do this is to actively look for what's best for you. Like in this case, you shouldn't be taking into consideration the prospects they have for you, that they never signed on paper. You should be regularly checking if you can change job for something that would be better according to your hierarchy of values (fe. better paid) and as soon as possible, change jobs or use occasion to renegotiate. And regarding renegotiating - again, healthy dose of paranoia - you don't consider anything that is not signed and on paper. If they for whatever reason cannot give you or even show you the chests filled with gold that they hide at the end of the rainbow for you, it's their loss, not yours. You don't have any obligation to avoid hurting their feelings with distrust. Do you know each other 30 years? Did you go fishing together when you were teenagers? No? Do they stand to gain from abusing your trust? Yes? See my point.