He/she, the actual one, still has the impression that you think of your ex, they may even think that you keep in touch and hide this from him/her. You make efforts to be better than the ex-girlfriend so you try to please him and ask him to tell you that talks about the exes do not need to appear in your relationship - there is place only for the 2 of you.
2. Money
She wants shoes; he wants new suspensions for the car. She wants a trip to Paris; he wants a mortgage for the house. Basically, talk about money will always appear, even if you want avoid this issue - either a partner is a spender or the other is careful with money, or maybe money is little.
3. Housework
One of you is very tidy; the other throws clothes all over the place, doesn't wash his/her dishes, enters with shoes on or does not want to vacuum. Also, one of the partners does not keep his/her word and responsibility is the other's problem.
4. The lack of time or distance
If you stay in the same town at a large distance from one another and do not find time to spend with your partner, there will be discussions. Even if the motives are real (the job, the boss, health issues) there will always appear a problem like ''you do not love me anymore and do not spend time with me'' or ''you found somebody else''.