Things won’t always work out the way you had planned. If you’re going after a big goal/dream and failing would basically devastate you, then it’s a good idea to have a backup plan just in case things don’t work out.
I know there are people who live by the all or nothing rule and think that having a plan B will only distract them from plan A but let’s get real here. Not everyone is going to succeed in what they set out to do.
It’s like driving around without a spare tire. You can be as careful as you want but often times, things out of your control will tend to mess up your plan and when that happens, you’ll only have yourself to blame if you don’t have that spare tire.
Having a backup plan isn’t the sign of the weak or undetermined; it’s the sign of the smart. If you’re afraid that having a plan B will cause you to not go all out on your plan A, then don’t make plan B so appealing.
Put yourself in a situation where if you fail on achieving your goal, your fall back plan will help you survive but it’ll also be unsatisfying enough that you will eventually go for your main goal again once you have the energy and the resources.