How do I stop overthinking? Am I being crazy for overthinking? Do I have unwarranted trust issues?

I am glad you have reached out for some clarity and support in this situation. First, it does not sound like you have unwarranted trust issues. One important thing to remember is that we are all shaped by the experiences we have in life - whether through things that are done or not done to us (commission vs. omission). Therefore, we experience the world around us differently due to how our brain has processed the information around us. Therefore, you are not "crazy" - it sounds like the experiences you have had in the past are impacting how you process current information whether it is warranted or not. 

The first step in understanding the thought process/overthinking is to recognize the thoughts you are having and look for evidence to support whether the thought is true or an opinion that is formed due to past experiences. It is important that when we feel we are overthinking is to be able to trust our thoughts. Not everything that pops in our head is true so we have to put it through some sort of test to see if it is true or opinion. Then if it is opinion then we need to learn to dismiss it through self-talk, "I know that is not true right now..." If there is evidence to say the thought is true (i.e. we see texts on the phone, we feel there is something more secretive things going on, etc.), you can validate the thought you are having and then it may be time to address it with your partner to get on the same page with these areas that will help move the relationship forward in a healthy way. 

During these times of overthinking, what you want to avoid is rumination - dwelling on the same negative thought over and over. When this happens, it can make the thought (especially an untrue one) feel very powerful and true. Thoughts can also often be intrusive thoughts. Having these is normal. However, when we "follow" the thoughts it can be unhelpful. Following the thoughts means that when a negative or untrue thought pops into your head, you play out the scenario of it and it can lead to an unhelpful and low place. 

As for your potential part of the question on what you should do - I would encourage you to better understand your thoughts and the background behind them. I would also encourage you to have clear and honest conversation with your partner to help move it to a place of understanding so you can establish trust in the relationship. At times in relationships we "miss" each other. Sometimes not intentionally doing something that harms trust but we aren't necessarily doing the thing that builds it either. It can be crucial for your partner to understand the true anxiety you are having over these types of situations. 

I hope this helps and you can find the support you need in these areas, either working on your thoughts on your own or through therapy. Support is always here to help.