Breakup 3 years ago over texts. Just saw her engagement party on Instagram days ago. Now what?

For what it's worth, I haven't been sad about the breakup in 2+ years and I've had her blocked for much longer. So seeing a random event coordinator's post on my explore page about her party ruined this isolation for me. I'm feeling myself fall into the pit again after a very productive last few months. I'm finally getting my life back on track this year and now it feels like it's falling apart. I saw a glimpse into the life that could've been had I just gotten my life on track when we were together.

At the time, we were both hurting from other things and never fully relied on the other, but there was a lot of love shared nonetheless. We both hurt each other in different ways as a result, and I have strong loyalty to people even if they hurt me. I know I wasn't perfect and I am able to appreciate how much I've grown since then, but seeing this video now makes me feel guilty and on the verge of breaking down and losing all the progress. "Why didn't I just be this person then". What can I do? I know not to drink the problems away, I know to continue focusing on myself in professional and physical manner, I have lots of trips planned. But there are moments in the last 3 days where I can feel the walls crashing in just like it was during my most depressed times and I hate feeling that way again after thinking I was done with it years ago. Comparison is the thief of joy I tell myself, but it's hard to not let the thoughts enter my brain anyways.