uneasy

hello everyone, this is my first post and im not really sure how the app works but I just wanted to share my feelings and see if anyone can offer guidance and opinions.

Me(20F) and my boyfriend(22M) have been together for a year and a half and have had lots of ups and downs. This is my first relationship so im not sure how relationships are supposed to work. I find myself wanting to know whatever he is doing and wherever he is going all of the time. I can imagine it gets overwhelming to him but its not easy for me either. About 6 months into our relationship I had checked his phone and seen that he was going to buy nude photos from another woman. He never did but ever since then I have never been able to trust him fully. It has caused me to question all of the small things and start arguments over nothing. I really love this man but I dont know how to stop obsessing over things I cannot control. I know that if it was him questioning me or causing arguments I would not stand for it so I know its bad of me to keep doing that but I dont know how to stop. I tend to speak before I think which also causes arguments and frustrations. It also feels like he is trying less and less for us. He used to buy me flowers every month and call me all the time and come over multiple times during the week. Now I barely get any of that. If anyone knows how to help or has any advice it would be greatly appreciated. Thank you:)