Needing advice

So my boyfriend D and I had been hooking up since July 2020 (kinda like a Covid hook up fwb, this worked best being long distance and crazy work schedules). Jan 2024 his grandmother passed away, Jan-March 2024 I had to work out of state but I still tried to be there for him virtually, when I came home I mentioned wanting things to be more serious, when I went back out of state May-June, he seemed more serious, what used to be communication every couple of days became daily, wanting to confirm I made it safely everyday on the road, he changed jobs to have a more defined schedule and ability to see me. We spent the summer really working on us, until I got the notification around the 10 yr mark of the night that changed me thar my abusive ex was up for parole. I figured the best plan at the time was to remove myself from earth, I thankfully failed, and realized I had some serious underlying issues that I needed to deal with before moving on to a new relationship. D was super supportive as I left for 2 months for an inpatient program and the 2 months of an outpatient program, as I was doing this he did an outpatient program to get over his past trauma too. Over the holidays, I moved closer to D and my family to have a better support system, April we made things official and the past 6 months were great to me regarding our relationship: his dad and my sister had both been diagnosed with cancer and we were supporting each other’s families, we both helped when his younger brother lost his job, we’ve been looking at homes to move in early 2026, we communicated but due to the nature of his work we didn’t see each other much during the week, he always kept up with my therapy schedule and knew my meds. Sept 7 my sister passed away, knowing death is hard for him I gave him a pass on physically supporting me. On the 18th I asked my cousin to braid my hair for the services which she did and I sat there and praised D as a great boyfriend, after leaving D called me and wanted to talk in person, we went to the park and he admitted that during our on/off fwb thing he had a fling with my cousin J (braider), during that moment I had very little capacity and minimized it that we weren’t officially together at the time. My sister’s services were on the 20th, D doesn’t do funerals well so he didn’t come and that Sunday D said he was too busy to spend time with me. This past week on the 24th we were supposed to hang out and he cancelled and moved it to the 27th. We hung out a little that morning talking and he said he felt guilty about not being there physically over the past few weeks, we both went to run errands and he’d be coming over that evening, it got to be around 9 and no word from him. As I tried calling him, my cousin started texting me for a sit down, after reaching D and explaining my cousin wanted to talk he finally told me that him and my cousin reconnected in January and had been hooking up twice a week until the 18th when my cousin realized we were together and told him to tell me or she would.

For context D and I had been friends since high school, so an 18 year friendship. My cousin and I are not close like we see each other at family functions but don’t talk regularly and D and I aren’t really on social media.

It’s hard because I love him and we planned a life together. Any advice would be greatly appreciated.