When dating someone you should speak with them about your intentions and what you expect from each other, part of that might be not sleeping with other people.
If you have been hurt in the past it may cloud your vision in the present which can make it harder to trust people, if you find it very hard to trust people then it may be worth seeing a counsellor so that you can process those past experiences and move forward with a fresher outlook.
But on the other hand, your instincts could be correct.
If you have valid reasons for suspecting your partner is unfaithful then you could always confront them and explain why you have concerns. Usually the last thing they want to do is talk about it so when you introduce the topic they may try to change the subject and steer the blame onto you in some way.
They may say things like “If you only trusted me more, maybe things would be better in our relationship” - but this is emotional manipulation, you should always trust your gut instinct.
If you have evidence they have cheated on you and broken your trust then most people would say you should leave them.
The following behaviour changes or reactions are some signs your partner could be cheating on you.
- They guard their mobile phone or other electronic devices
- They are emotionally distant they are suspicious of you
- They suddenly start improving their appearance
- There are periods when they are unreachable by phone
- Less sex in your relationship hostility towards you
Please be aware that your partner may show some or all of these signs and not be cheating on you, the only way to know for sure is to have solid evidence.