Hi @Nikuukin
Welcome to the forums
I'm sorry to hear that you experienced that in your past relationship, and it's completely understandable that this would be playing a part in your difficulty trusting in your current relationship. It is interesting that you noticed that you had struggled with trust prior to your past boyfriend. It can be really hard to get to the root of things like this, and often it's something that therapy can be very helpful for as you and your therapist can really delve into what may have happened to cause you to feel as though you can't trust those around you. Is that something that you might consider?
Otherwise, it might help to reflect on the relationships you had and saw around you when you were younger - often what is modelled to us as we're growing up can form deep-seated beliefs about the world. For example if you saw lots of turbulent relationships where trust was an issue it would be understandable that you take that in and have it affect your ideas around relationships.
If this resonates with you and you wanted to go a little deeper, you might be interested to look into attachment styles and how they can be influenced due to our early experiences
Whether or not that feel relevant to you, it's clear that you really care for your partner and it's great that you're willing to recognise that this is an issue and something that you're wanting to work on, and reaching out is a great first step