Relationships operate under the premise that the person you choose to be with has positives that you love and negatives that you tolerate. In the honeymoon stage, many things that are inherently annoying, frustrating, or inappropriate get ignored because you're smitten. But the "feeling" of love fades. After some time, you're left with the responsibility of giving love. There's now a daily investment to maintain an even balance so the feeling doesn't dissipate. However, in every relationship, there will come a time where you will have to say something to your partner that's hard for them to hear.
Just because what you've done in previous relationships has worked doesn't mean it's going to work with me.
Criticism is never easy. People automatically will go on the defense when they're being told something they're accustomed to doing is no longer acceptable. Valid issues may get reduced to making you feel guilty because for your partner, you're the only one who sees the issues are problematic. Nobody likes to have their shortcomings pointed out to them. Even more importantly, nobody wants to feel like their way of doing things offends or hurts the person they genuinely love.
There are a few different approaches you can take if you want to broach the subject of relationship criticism. Whatever the grievance is, the key to giving constructive criticism to your partner is to understand that the end result is to come up with a solution, not attack the person.
Example: You work 16 hour days. Your partner is feeling like he/she has to struggle to get couples time. And when you do spend time together, you're not really 'there'.
As the person who is giving constructive criticism, you should understand the variables involved. Can your partner leave work earlier? Can you fit in a couples' lunch away from the office? Is the intensity of work situational or permanent? These are all things to ponder as you prepare to have the talk. A conversation about how you'd like him/her to be home more often doesn't need to escalate a fight about his/her work being more important than the relationship. Defense and deference will be the result because that approach will be looked at as complaining.
On the receiving end, it's important to remember that the person you love doesn't become an enemy because they don't like something you do. Even if you've had several relationships where you working 16 hour days wasn't an issue, that doesn't mean that your current partner is clingy or needy. It doesn't mean that something's wrong with them because they'd like more. It means for that relationship to be viable and functional, you're going to have to compromise. When you love someone, uncomfortable conversations and criticism are opportunities to figure out a better way to do things so both of you are happy.
There's a fine line between constructive criticism, complaining, and nagging. You owe it to yourself to stick the standards that you had when you met the person. People can't always magically change overnight. When someone has done something for so long and never been told it's wrong, there's going to be resistance. They may even outright deny that your claims are credible. The fact of the matter is you're not his/her exes. Your opinion and perspective is unique. Therefore, the criticism you give will either prompt self-reflection that your partner is overdue for or create resentment that will eventually permeate into other areas of the relationship.