What do I do about my relationship? (Throwaway)

To give the most amount of context, me and my partner (both 19 years of age) have been together for a little over a year and a half. We got together shortly after the both of us got out of other previous relationship. The relationship I was in previously was pretty serious. The breakup not stemming from a loss of interest but more so our surroundings if that makes any sense at all and me and that person were also previously best friends. I’ve always still had some sort of small feeling towards that person even during my current relationship admittedly. I’ve always struggled mentally as well. Depression, anxiety, stress. Not excuses, I just think it’s important to express that I get really conflicted with my thoughts sometimes. I’ve also always struggled with lustful thoughts towards other people even when in a relationship. No acting on it, just thoughts. To make a long story short, my current relationship is honestly extremely good. They are extremely supportive, respectful, and treat me better than honestly anyone ever has. They have been here for me through almost everything. That’s why I’m struggling with what to do. I feel stressed being in a relationship. I feel stuck and honestly I’m not the most supportive partner I could be. I could be better and I feel that I’m not the person that can do that for my partner. My ex contacted me the other day, just normal “how have you been” talk but those small feelings came up again. I just feel wrong. I feel like I don’t know what is best for me or my partner. A part of me doesn’t want to be alone and really does love and care for my partner but I don’t want to be confined to a relationship. I’m not sure why that is. I feel wrong feeling this way especially because my partner has done nothing wrong but I feel like I’m not a good partner fit for anyone right now especially since I still have those small feelings for my ex, conflicting thoughts on being in a relationship, and barely any time to give to my partner. I’m not trying to leave my partner for anyone else. I just know I’m not the partner I could be. It’s been expressed multiple times that they feel they contribute more than me. I just don’t think that being in a relationship may be what my heart is pulling towards right now. Can anyone help me? Sorry for the rant. I just have a lot on my mind. I don’t want to be a bad person to them or anyone. Thank you for listening.