When I have anxiety about my relationship why is it so crippling/overwhelming?

Been together for 2.5 yrs. I am her first “real” relationship. Aside from 1 high school fling. I am also her first girlfriend. She has had very limited sexual encounters 3 including me. She mentions how she fantasizes about people we know when she pleases herself and sometimes when we are having intimate moments, and she said she would never hurt me by cheating on me but sometimes it’s difficult to be faithful but she chooses me over the temptations. I feel like I’m overthinking because she said she wants to be with me. But I feel like this is not okay.
Asked by Baller22

This is a tough one! I have a few thoughts regarding what you shared with me. The first thought is on the fantasies that you mentioned. I think it is very normal for someone to have fantasies about different things, including other people. It is very common for women to have to, or want to, fantasize during intimate moments. I understand that it must be difficult to know that your partner is thinking about other people. On one hand, I think it says a lot how much your partner trusts you that they shared that private information with you. On the other hand, I am sure you would prefer that they are fantasizing about you. As challenging as it may be, I think it is important for you to check in with yourself and decide if you do in fact trust your partner that these fantasies are harmless, or if you do not trust your partner.

Now, the second part is your partner stating that it is difficult to be faithful. This is a bit more concerning. Once again, I am thrilled to hear that your partner is being open and honest with you, however, if you are not in an open relationship then faithfulness (I assume) is extremely important. Your partner stating it is difficult to remain faithful (I am guessing) makes the fantasies that they are having feel more worrisome. Have you sat down with your partner and asked them to be very clear and specific regarding their struggle with remaining faithful? Are they wanting to be with other people? Have they come close to being with other people? Having the full picture, I am guessing, would allow you to then assess the situation for what it is and then make a decision based off of that information. 

I am not trying to say that one of you is in the right, and one of you is in the wrong. What I am trying to say is that I think what you are sharing indicates that a larger, more serious, conversation happen between the two of you so that you can assess what next steps make the most sense for the both of you.