Bonus received in 2019 but included on 2018 W-2

All,

 

Help is appreciated. My employer surprised me with a sizable bonus in 2019. Check dated 1/4/2019. Received and cashed in 2019. I'm grateful for this bonus. However, they added the bonus to the last pay period of 2018 and included this check amount in my 2018 W-2. In their mind, this was an "end of year" bonus and treated it as such. This bonus, when applied to 2018, pushes my AGI just beyond the amount that Health Care Marketplace financial assistance supports, and full cost of entire year of health plan becomes due. (I was working very hard all year to stay within AGI limits). Without this bonus, I gain a large tax refund. Many attempts to get amended 2018 W-2 from employer have failed. I'd like to have this bonus applied to 2019, the year I actually received it. Can I file an amended return for 2018 in any way? Do I need advice from a tax attorney? Just trying to get an idea of next steps. Thanks for any advice given here. Much Appreciated!