Adjusting to atheism can take a while. For me, the initial adjustment was quite quick and a lot less painful than I thought it would be. I suppose the thought of spending the rest of my life knocking on doors and devoting all my spare time to JW pursuits was more painful than the idea of having my life back and dying in a few decades. Even so, going from "I'm gonna live forever!" to "I'm gonna die!" is never going to be easy.
After accepting my own mortality, I still struggled, in fact, at times, still do struggle, with the feeling that I am wasting time, or that whatever I do is futile as I'll be dead soon anyway. There are some things I find helpful in dealing with this - whether they will be helpful to anyone else, I don't know, but maybe you will find your own little helpers as time goes on. Here are some thoughts that help me:
1) When my wife's grandfather discovered he had terminal cancer, he said "Oh well, I've had a good life." - and he seemed very calm, almost content ('old and satisfied with days' you might say). I aspire to be able to say the same thing when my time comes.
2) Having thought about that a bit more, I realised that already I have had a good life. If I die today, I have had a much better life than the vast majority of people and animals that have ever existed. I have had some amazing experiences, and I cherish those.
3) The alternative to facing up to the reality of my own mortality is to deny it. If I do that, it might make me less anxious, but it also causes me to value my life less and waste the precious little time I have available. Knowing that I only have a short time at least lets me prioritise better and make the best use I can of the time I have. Denying the fragility of life also means denying its value.
4) I have started going to philosophy meetups, and this has helped me to view things, well, philosophically! I used to obsess over the fact that the present moment is in the past before I've even finished formulating that thought, that whatever I am doing right now is pointless because it will be over soon, that the future is just the past waiting to happen, that the rest of my life will be over in a flash, etc. But philosophy has helped me to see that the present is something you take with you - it is never in the past, it is always with you, and always will be. The present moment is the most important thing, and being attentive to it, excluding external irrelevancies can enable you to find a sense of contentment that is not dependent on other people, on the past, the future, or events outside your control.
5) Figure out what activities and pursuits bring you the greatest sense of pleasure and accomplishment, and prioritise them. This is not always easy, but if you can get into a state of 'flow' - where you are absorbed in an activity you enjoy (playing an instrument, riding a motorbike, even watching a gripping TV show), your level of satisfaction with life will increase.
6) Even when you are not doing something that you especially enjoy, being mindful of your surroundings and appreciating the natural world can also be therapeutic.
7) Try to find ways of helping others - donate to charity, do some charity fundraising, volunteer, even just answering posts on a forum like this.