You don’t have to tell them everything. You are allowed to keep things to yourself. You are a grown-up (most likely). If you don’t think they can handle it or it is something you prefer to keep to yourself, that will forever be your call to make. Unfortunately, that also means the relationship will likely never reach its full potential. You don’t have to trust people, but without that trust, the relationship will always be functioning at a disadvantage.
Trust is one of those things that you will never know for sure. It is a leap of faith. Short of being able to read minds, and don’t think I haven’t tried, it is impossible to know for sure whether or not anyone is being completely upfront and honest with you. Luckily for you, the same goes for everyone else as well. If you view it as a disadvantage, that is what it will be. I recommend viewing it as an advantage.
You can go through life trying to do everything on your own, but you don’t have to. Relationships are about making that connection with someone that is based on love, respect, and mutual trust. Having the ability to trust someone fully allows you to face anything in life knowing that there is someone that knows everything about you and still has your back. Is ready to support you and keep you focused on the things that will make your life together better.