How do you deal with your (adult) child's decisions when you don't particularly agree with them?

All you can do is guide and advise them...you can't live their life for them...
I can imagine it's difficult when you really don't agree with them

They must be allowed to make their own mistakes, it's the only way for them to learn.

I wouldn't want anyone telling me how to live my life, so I'll let them make their own decisions because it's nobody's life but their own. I will respect and support their decision.

if they are not living under my roof, then it's up to them..and I told them that as well.. "the decisions you make will be yours, and whatever you decide, YOU have to live with"...

An important distinction to make is... are you projecting yourself onto your child?

What I mean is, are you living vicariously through your kid as opposed to letting your kid grow up and experiencing life the way they're meant to experience it?

I'm not saying it's ok for them to do terrible things like crime and drugs.

What I'm saying is, if they're making a decision that may not lead to a good career or maybe they're marrying the wrong person or something, I don't know... are you supportive of them regardless of the outcome?

Are you going to be able to be the parent that will provide them the support they need if they realize they failed?

Forging that emotional bond and talking with your child and giving your perspective, that's ok. Just don't force them to do anything. At the end of the day, offer that advice and leave them to make their decision regardless of the outcome.

When they realize they made a mistake, they will think of you and your emotional bond will be stronger because of it. They're not going to grow to resent you, they're going to grow to respect you because it's a lot easier to love and appreciate someone who is there for them as opposed to someone who forced them into a decision. They're going to make a mistake based on YOUR decision, not THEIRS. Hence, resentment will be against YOU as opposed to THEMSELVES.

As much as it hurts to watch we have to let them learn from their own mistakes. 🙁

Sometimes they just need to learn the hard way. When my daughter was 21 she had a 33 yr old boyfriend. Eventually she needed a bit of assistance to get rid of him because he turned out to be a prick. She now has a boyfriend closer to her own age and it's going much better.

I think you have to give your opinion and whether they choose to take it is up to them. I have no doubt your a good parent and only want the best but it can be frustrating.

If they are adults and can make their own decision, they are old enough to be criticised for those decisions.

Try to guide him/her the best you can even though you feel like banging your head against a brick wall is less painful and that the wall is more receptive to the advice than the kid.