I am always scared of being cheated on in all relationships, how do I cope with my partner participating in activities without me?

Trust is something really delicate. And though we maybe should, we don't always trust our new relationships as openly as we could after having a bad experience. You need to decide if you think that your trust issues stem from the past or from the actions of your partner (if they have broken your trust in the past). From there you will know what it is that needs to be worked on so you can have less anxiety in the future when doing things separately.

Your past relationships are not the present ones, it’s going to be hard to face your fear, but trust yourself; you deserved to be love. The ones who left did not deserve your love, you are much more braver than you think if you do brace yourself for another relationship. Take it step by step, open yourself up to new possibilities! You got this!