Assalamu alaikum wa rahmatulahi wa barakatuh,
It sounds like you are carrying a lot of guilt around for your past sins and this is continuing to make you feel continually depressed. However, Allah is the most Merciful so you can trust that there is a way out of this. Stepping up and admitting your problem and seeking help as you are doing here is an excellent first step in overcoming your difficulties. Masha Allah you also say you have a supportive husband and this will also be a huge help in your recovery.
The first thing to do is to go back and consult the root of your problem, which in this case appears to be your involvement with another man. People who have low self-esteem as you report will often find ways to boost their esteem to make them feel better about themselves.
One way this can be achieved is to get the attention of a member of the opposite sex. Since you got involved with another man, it seems this might be a way in which you unconsciously sought to have your self-esteem needs met. Unfortunately, this has to lead to even worse consequences as you now have to face the guilt of having committed a sin and betraying your husband.
Remaining on this aspect alone, for now, it is suggested that you find alternative ways to boost your self-esteem. This way you won’t feel such a need to do it in ways that are not permissible and cause ongoing harm to yourself and others. Take some time to think about things that you are good at or even something that you want to improve on that you just like to do.
Take time out each day for yourself to engage in this activity, whether it be some kind of sport or recreational thing, or something like studying or improving your Qur’an recitation skills. Either way, this will give you time to yourself each day to do something that makes you feel good. Set yourself weekly goals with this activity to give yourself a sense of accomplishment as you achieve these goals.
This sense of achievement is a good way to boost your self-esteem in a positive and halal way. This will also then prevent you from seeking other ways to have your self-esteem needs met that are less favorable.
Alhamdulilah, your husband was understanding and has stood by your side and been supportive. This is a sign of a good husband, which you know as you are happy with him and love him dearly. However, you do also say that he is not as romantic as you’d like him to be. It may be that this is also a contributing factor to your poor self-esteem too as you are interpreting this perhaps to mean that perhaps he doesn’t love you as much as you would like for example.
You a need to understand that there are many people, both men and women, that just simply aren’t inclined towards romance, or at least to be the initiators of romantic gestures. This does not mean that they don’t actually love anyone, it is just a trait. Likewise, it also doesn’t necessarily mean that they don’t want to be romantic. It would just be that their shy nature makes them feel uncomfortable to do so.
If this is the case then there is no harm in you trying to initiate the romantic gestures. Once he experiences it and comes to love it, he may surprise you some time and be the one to be romantic towards you.
Most importantly, amongst all this, you can ease your worries with the remembrance of Allah. With the primary issue here seemingly being your guilt about your past, begin by sincerely repenting to Allah and asking for His forgiveness. Allah loves to forgive and loves when people repent to Him.
Allah is the most Merciful, so it is important that as you ask for His forgiveness, you trust that He will forgive you. This will also help to relieve the emotional burden that you are experiencing. Getting close to Allah generally will bring you comfort as well as developing a fear of Him that will prevent you from behaving in ways that are less favorable again in future.
May Allah ease your burdens and bring you comfort. May He bless your marriage and may you find peace and contentment in His remembrance.