Should I continue this relationship?

Hello. I am writing here as I, honestly, do not know who else to turn to. I believe my boyfriend had feelings for his ex while we were in a relationship, and there are some pretty telling messages...

Me and my boyfriend met in 2019, and have been together ever since and, honestly, our relationship has been a mess and I am only now coming to terms with all the boundaries that I let be broken without doing something at the time. Now I feel like I am completely stuck. I love him, and leaving him seems like the hardest thing on earth to do. I just need to hear peoples opnions on this, as I feel like I am going crazy.

We met in 2019 while working together, and after knowing each other for 2 weeks, we slept together. Problem was: we were both seeing other people at the time. Not in a relationship, but romantically seeing other people. Before we even kissed, I sat down with him and asked him about the girl he was seeing and if he was sure he wanted to get involved with me and what he would do, to which he replied that "things with her were not going anywhere". I also told him about the guy that I was seeing. After he told me that things with her were not going anywhere, we got involved, as I took that as a way of him saying that he won´t see her anymore.

After that, we spent 2 weeks working together and were envolved during that time, and after that I had to go back to my home country, I told him he should come and visit me sometime, to which he replied he would love to. But, a few weeks after that he messaged me saying he was going to spend two weeks with the girl he had been before. ( She had invited him a while back to go and see her at the end of summer, she invited him in May of 2019, and me and him met in July 2019).

I am honestly not sure what I though at the time, but I think I just thought we were going to continue living our lives and seeing other people. They slept together on the first night he arrived. He says it was a mistake and that he got too drunk, and that he didn´t know how things would go with me. That he was terrified that I would reject him if he told me how he really felt about me, and he was also scared that I was still seeing the guy I told him about when we met.

He said that on the second day, when he woke up, he regreted what had happened and that the same day he told her about me that he had gotten involved with me and that he liked me and wanted to see how things would go. He says that the rest of those two weeks they were there as friends, so he basically broke up whatever it is that he had with her.

Fasting forward, I always knew they were still in communication, as they remained friends like he said.

However, I have found out recently, that in 2020 he sent her a book with a letter. That book costs around 100 euros...So I snapped and asked him to show me the messages from 2019 with her.

She sent him a message thanking him for the gitf saying "it was the best thing she received in her life, that he is the only man who has touched her heart, that she has missed him so much from her days, how his presence just makes her days better and how he completely knows her heart" to which he replied that she knows his heart too, and very few people do, that she understands him and very few people do and that she could say more to him with one look than one hour conversation with a lot of people. Also in 2020, he said to her "would be great to speak again soon, I do miss your voice".

And this is not all, I guess at some points in the beggining of our relationship I felt insecure with him having a friendship with her so at some point in 2021, he mentioned her a few times and I freaked out and asked why and if he had any feelings, to which he replied he didnt. A few weeks after this, he told me he loved me for the first time when we got drunk at a party and he said that "there was still some attraction there for her". I confronted him the next day and he says that is not what he meant, that he meant attracton has in attracted to her as a person, as a friend. So I have been quite traumatised about their relationship since this one specific incited, ans this has led to me asking to see the messages.

I know this is a super weird timeline, and probably a confusing story to understand. My biggest fear is that he had feelings for this person while he was in a relationship with me. I am absolutely shocked with these messages, I never thought they communicated in this way and I honestly dont know how to go on. I think I also have developed or have flared my OCD and I have repetitive, intrusive, constant thoughts about this every day. My menatl health is declining to a low point that I have never experienced in my life. I honestly just feel so stupid, I feel like I should have not even gave him a chance the moment after he met me and decided to go see her for two week, that I should have left him when he said that there was still some attraction there for her.

I have undermined my feelings, I thoiught that I was beeing crazy, I thought why would he be with me if he still had feelings for her? But now after reading these messages? They seem romantic to me. It does not seem like what friends would text to each other.

We have been talking about this for months now, especially since I found out about the book and the value of the book. I honestly do not know what to do. He swears on my life, he continues saying that he did not have feelings for her, that he went to see her those two weeks with the intention of breaking things of with her because he wanted to be with me. But I just keep having this feeling that he had unresolved romantic feelings for her, as it is the only way I can explain these messages between them, it is the only way I can explain that he would go out of his way to send her a meaningful, expensive book, with a letter. While at the same time, he did not sens me anything at that time.

He says that for a long time in our relationship he was extremely insecure that I would leave him, that that is why he also took a long time to tell mehe loved me and that m,ight also be the reason he would focus on friends more. But i just dont believe it. Who is in love with someone and decides to send an expensive gift to an ex?

I feel like during our relationship I overlooked things because I truly loved him, but now I just feel like I deserve something better. I know he truly loves me, that is not even a question, but I doubt his feelings in the beggining of our relationship, I doubt what his intentions were with me.