I feel this. It's incredibly painful to make that kind of realization and it's hard not to be resentful about it
This happened to me and I was bitter about it for about 2 years, should've been in counseling but didn't even realize I was bitter
Another thing you can do in parallel is reaching out to other friends/acquaintances you might have lost touch with. Tell them honestly what you appreciate about them and that you'd like to revive the friendship.
In both cases you'll feel awkward and uncertain. It's normal. Also, don't expect any single relationship to fulfill your unmet needs. But making yourself vulnerable to others is the beginning of "working through it yourself".
How did you do it? Have you created an Excel spreadsheet and questioned them about their degree of effort, which you then compared to your level of effort? You added everything up and your column was higher than their column?
I went through this approximately a year before you. Loneliness in this period is caused by your slow response to external change, as well as life telling you to make a life for yourself.