I'm not sure if this might help, but there are three things I've noticed can make things easier.
1 - Trust yourself. As much as the word gets bandied about it means different things to people, and what you define as trust might be different for the other person. What does that have to do with trusting yourself? Put simply, if you're acting in good faith and trying to have real relationships with people (well, even superficial ones), you need to be aware of what it is you're looking for. To follow from that, it doesn't really help so much to keep beating yourself up for having made a mistake in trusting someone who wound up not being trustworthy - you'll have been acting on the info you had at the time, and made the best decision you could., Even if it was a sense of "I knew it was a bad idea" - well, that means you knew it was a bad idea but you decided to give them the benefit of the doubt anyways, which, for someone who struggles with trust, is actually quite a leap. If you're someone like that, don't ignore this, and don't ignore whatever instincts, signs, or feelings you get about someone. Sometimes those aren't necessarily super clear but I've found that (at least in my case) the "personal radar" can be quite useful if (and this is a big if) you know how to interpret what it is saying. Sometimes it works out and that gets validated, and sometimes it doesn't and you learn that what you saw isn't what was there, but hopefully you can take that new info and use it for later - slowly getting better at trusting your own decisions, actions, and instincts.
2 - To follow on from 1, you need to be willing to trust others. Admittedly this can be a really big hurdle to cross, but you need to be able to remember that not everyone will treat you poorly, and while assuming that they can (or will) does have its advantages, it does admittedly close you off to other people that might actually be the kinds of people you want to meet - people who do enrich your life and make you glad you met them. Not saying you have to jump in at the deep end and give your whole life's story or tell them your most precious secrets, but if you can dip your toes in a way you're comfortable with, it can slowly start to compound as you go further and further into trusting others, while using the resulting info to be more and more aware of how your efforts are received. Again though, and this needs to be emphasized, you have to be able and willing to do this when you're ready and willing to take the gamble of it not working. Hoping for the best, expecting the worst, and being willing to keep trying when things don't go the way you hope they do. If you're still hurting from whatever situation caused you to lose trust then that's going to be a big ask, but hopefully, even if slowly, it won't be as big of an ask anymore. It can be baby steps, too, and trusting someone to show up for your lunch meeting like they promised is not nearly as big of an ask as asking someone to treat your deep and dark secret about (random example) you gambling $1000 every weekend while struggling to keep up with your bills seriously and not think worse of you. This really is aided if you've had positive experiences with trust though, and having one, two, or more people you do honestly trust still, and can talk to, would help a lot. It's not always an option but you *can* rebuild by yourself, though I imagine it will take quite some time longer to do so.
3 - Set (and declare) your expectations. I've noticed that trust is / can be broken thanks to you having different expectations from the other person. Maybe they want to be closer to you than you want to be to them, maybe you're hoping for something they can/t / won't / don't want to provide, maybe something else. This sin't going to help all the time, but if you can be clear with your expectations and concerns (whatever they may be), it lessens the chances for miscommunication to occur. Say for example you ask the person to call you every week at 8PM (random example) - if you tell them this and they make the effort, it's going to show that they're trying to live up to that. If they don't, it'll show that they won't. If they do but get sidelined by an emergency, or something else, it makes it easier to see what happened and how to move forward. All of these are alternative (and, in the sense of relating to one another) more constructive options that leave open the opportunity to move forward in a way that suits you (and presumably them). Contrast this to not telling them anything, expecting them to call at 8, then being upset when they don't, then them turning around and telling you that they had no idea they needed to call at 8. Sure you might be able to patch over that but it might make you more wary of them (and them more wary of you) regardless of that patch effort.
I'll also add in one extra thingy. There's a book I've been reading called "Daring to Trust
" by David Richo (would highly recommend this - it can be difficult to read in parts as he pushes you to be honest about yourself and your actions and confront things you might not know you're holding on to, but he makes a lot of good points: several actually. One of the points, paraphrased, is that trust has to be based on a repeated history of consistent behavior and actions, and not your aspirations or (unfounded) beliefs about the other person. It's something that isn't necessarily thought about often - usually only when the trust has been breached.
I'll posit, too, that even if you don't believe that you have trust (at all), it is still present. The Japanese have a saying, "itadakimasu", which apparently means multiple things - one of which is an expression of thankfulness / gratefulness for a meal, and those who made it possible to eat that meal. I might be wrong here or not providing the meaning properly, in which case someone can correct me, but to continue, it does show something in this context - namely that if you buy food from a grocery store, you'e showing trust in the store to be providing you food that is safe to eat and not rotten or old. You're also trusting the government in both making sure that that food is safe (ie food inspectors and licensors), and that the money you pay will be accepted as valuable by the store (ie they'll let you take the food for the money, where they might not if you used, say pounds or yen, or euros). You're trusting the farmer who grew the food (if vegetables or meat) or the factory worker who packaged it, etc. It's not something that gets thought about often, but that *is* a form of trust, and indeed it is vital, for you can get very sick very quickly if the food you eat is problematic. If you're willing to trust these entities, some of which you haven't (and never will) met before, it can be possible to trust other people / entities that you can meet, learn more about, and draw close to, both physically and metaphorically.
I don't say any of this to suggest that it is easy, or fast, or even going to be particularly fun - it can be very very much the opposite of those things, but it is indeed possible to come back to the point of trusting others, as well as yourself.