How the f*ck do you do estimates?

I have ~7 YOE and was promoted to senior last year. I still have a really difficult time estimating how long longish term (6 month+) work is going to take. I underestimated last year and ended up having to renegotiate some commitments to external teams and still barely made the renegotiated commitments (was super stressed). Now this year, it looks like I underestimated again and am behind.

It's so hard because when I list out the work to be done, it doesn't look like that much and I'm afraid people will think I'm padding my estimates if I give too large of an estimate. But something always pops up or ends up being more involved than I expected, even when I think I'm giving a conservative estimate.

Do any more experienced devs have advice on how to do estimates better?