Zina is not penetration only. There's different kinds of zina. The zina of eyes, which is looking at things one should not be looking at (porn, etc.), the zina of hand, which is touching that that is not allowed (masurbation, etc.), the zina of tongue, which is talking about all the unlawful sexual stuff one desires and then the zina of the private parts, which is penetration. The last one is the major zina and is punishable by Hadd under Islamic Law-- assuming all the requirements are met.

It was narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah has decreed for every son of Adam his share of zina, which he will inevitably commit. The zina of the eyes is looking, the zina of the tongue is speaking, one may wish and desire, and the private parts confirm that or deny it.”

Narrated by al-Bukhaari, 5889; Muslim, 2657. ^[1]

You should seek forgiveness from Allah. Try to be fearful of Allah (SWT) both in public and private. Offer the five daily prayers on time. Allah loves those who repent. It's mentioned in Surat Al-Baqara verse number 222:

ويسالونك عن المحيض قل هو اذى فاعتزلوا النساء في المحيض ولا تقربوهن حتى يطهرن فاذا تطهرن فاتوهن من حيث امركم الله ان الله يحب التوابين ويحب المتطهرين

And they ask you about menstruation. Say, "It is harm, so keep away from wives during menstruation. And do not approach them until they are pure. And when they have purified themselves, then come to them from where Allah has ordained for you. Indeed, Allah loves those who are constantly repentant and loves those who purify themselves."

emphasis is mine.

For more information on repentance, please see this article on repentance. ^[2]

What you did is considered minor sin. In Islam Adultery happens when a penis penetrates the Vagina.

There is no punishment for such actions in Islam, which are forgiven by doing your daily Salah and asking Allah for forgiveness.

There is a similar hadith ^[1] about the prophet PBUH for your situation:

It was narrated from ‘Abdullah bin Mas’ud that a man did something with a woman that was less than adultery; I do not know how far it went, but it was less than adultery. He went to the Prophet (ﷺ) and told him about that. Then Allah revealed the words: “And perform the prayer, at the two ends of the day and in some hours of the night. Verily, the good deeds remove the evil deeds. That is a reminder for the mindful.” [11:114] He said: “O Messenger of Allah, is this only for me?” He said: “It is for everyone who acts upon it.”

Don't get me wrong, what you did is still considered Zina, but the type of Zina that does not come with Punishment (looking at porn, Lesbianism, masturbating, etc..) and erased by asking for forgiveness.

Personally, I would think it is OK to get married without telling your future husband about this issue, since it will cause more harm than good. When Allah covers your sins, you shouldn't broadcast them to anyone.