Developing feelings for someone while trying to move on from another?

It's been a year since my four-year relationship ended, and every day has been a challenge. I still can’t believe how long it’s been. The breakup was profoundly painful, and not a day passes that I don’t think of him and the sadness he left me with. I've prayed countless times to move on and forget him, but it's proven difficult. In the midst of this, Phil, a high school friend, came back into my life. We had reconnected before the breakup, but after everything crumbled, he became my lifeline. We bonded over our shared experiences of heartbreak, among other things. He became my pillar of support when I didn’t feel I could rely on anyone else. The loss of mutual friends with my ex only made Phil's presence even more significant. As time went on, Phil and I grew incredibly close. With him, everything feels easy—comfortable and safe. We share so many laughs, and it’s been a long time since I’ve felt that kind of happiness. But now I find myself developing feelings for him, and it frightens me. Given my past, the thought of opening my heart to someone again, even someone as great as Phil, feels risky. I’m terrified of losing him too. I don’t plan to reveal my feelings. The fear of losing yet another best friend is overwhelming. But at the same time, I’m unsure how to handle this situation. Despite all the pain my ex caused me, a part of me still feels attached to him, and I have no idea if Phil feels the same way about me. I’ve started to distance myself from Phil out of worry. My thoughts are all over the place. What should I do?