How can I say something back without seeming desperate or offending someone else?

Saying it in a politely way. Respectful and serious. That is very important. If you say it in a good way, you won't seem desperate

Express your opinion! That's important. You won't seem desperate or offending, it's just your point of view. People will respect that.

TONE. it is in the tone of your voice. studies shows that people tend to be angry at someone in the way the talk. just be cool and level headed.

Just keep it simple, with simple words too, when we trying too hard, it shows!always be kind, and mostly, say things clear as possible

Think it through in your head first. If it sounds rude to you and is a judgmental thing don't say it. Phrase what you want to say in the nicest way possible.

I suppose this depends on what you want to say and who you are saying it to. Usually, prefacing a statement with the phrase "with respect" goes a long way towards alleviating any potential offence but the situation and the person dictate what you say. A recent example I can relate to is persistent locking out of my work website. I keep the complaint and the person separate by saying "I'm getting locked out of the system every week. I realise this isn't your problem but as my manager I'm sure you can imagine how frustrating this is for me. I'm enjoying working here a lot and these constant ICT issues are spoiling the experience."

Don't worry about what other people see you as. Your mind is the most important thing to you. the only thing that matters is how you see yourself.

everyone's problems are unique so I won't say I know what you're going through but I can understand

Be honest with yourself. If you said that same thing to someone you cared for (think grandparents) would they be offended. If there's a slightest possibility I would try to rephrase it in a way that you won't think is offensive to anyone.

Say what's on your mind! Just sugar coat it a bit so they know your intentions were not ill. Also, address yourself as not trying to be rude.