If a great opportunity falls in your lap, but it'll be tricky to bring off successfully, see if there's a way you can give it a try while maintaining an escape plan. If so, set up the timeline for when you will definitely proceed with it or will use that exit. Then go for it.
When you're trying to make a project come together be sure to regularly weigh the work required, time remaining and chances for success. If the picture gets bleak, instead of exhausting yourself for a poor result, choose one of these options
– scale the project back to make it achievable with the originally planned amount of work in the time remaining,
– extend the deadline to allow you to fit in more work at a non-stressful pace,
– decide it will not come out well enough to be worth it, cut your losses and call it off.
As time goes on you'll get better at estimating the work required, calculating the time things take, and judging before commitment whether something will be rewarding. If you let yourself intelligently make some mistakes by taking some risks, you can learn from them.