How should I decide to be in/out of a relationship?

Quite easily. Go away somewhere to think it out and don't be 'guided' by anyone else. On your own you can think clearly what the relationship means to you and you can list the negative effects of staying, and then of ending the relationship.

You may worry that you will not find a partner who suits you so well, but if that were so, why would you be querying it now? Plenty of friends and family might seek to advise you, but only you know what the relationship holds for you.

As for the losses, you should not consider minor conveniences such as 'I won't get a life to work" or 'My Mum won't have anyone to shift heavy things'. You must look long term. Do you want to be together when you're 40?

There are no criteria to determine if you should be in or out of a relationship. This depends on your interest, level of maturity and the commitment you are ready to put in to it. Here are things to consider before stepping in or out of a relationship.

Consider your motives: do you think you want to be in a relationship? If yes, why do you want to go into a relationship? And if otherwise, consider if your reason(s) is/are genuine. Know that you don’t go into a relationship because your friends are in a relationship or because you feel like jumping into one.

Consider the demands of a relationship: you must recognize that a relationship will require certain things from you. You need to be sure that you are ready to give your time, effort, feelings. Ask yourself if you are okay with being exclusive with one person.
Hope these tips help you.

Whether you are better in or out of a relationship rather depends upon what you expect of it. This question could be answered more helpfully if you specified whether this is a boy/girlfriend relationship or just a friend of the same gender.

Lets say you have enjoyed hanging out with a particular guy/girl recently and a re wondering whether to take this friendship more seriously. Presumably the other has asked for this, perhaps for some commitment? Now if you like being one of a crowd, it may be that this relationship will prevent you enjoying the crowd scene or nights out in a group. Consider this carefully.

Will committing to a relationship mean giving up a lot of fun that you really value?

On the other hand, if being a twosome is the most important to you, can you happily give up on being an active part of a group? Or is your question more about your doubts about a relationship you're already in? Is s/he making demands on you that you are finding it increasingly difficult or uncomfortable to meet?

Then be bold and make yourself clear to the other. e/.g. 'I am happy to give up going out with another partner for you, but I am not willing to .... ' whatever it is that is making you wonder whether you should be in or out of this relationship. The younger you are, the more careful about commitment you should be. Time enough for that after age twenty or later.