"If there are two verbs in a sentence the second one should be in the infinitive form" is certainly not a rule that I would teach. Make that "in some verb phrases a verb that is usually conjugated is followed by another verb that is in the infinitive form", and you'll have something that does work in some specific contexts, with some specific verbs.
For example, it works in certain propositions where the main verb is poder, deber, querer, tratar, creer, and others, some of which an English speaker would analyze as modal verbs (can, should, must) or as verbs that often take a subordinate verb (want (to), try (to)).
- No puedo creer lo que escucho. = "I can't believe what I'm hearing."
- Creo entenderte. = "I think I understand you."
- Deberíamos dejarlo solo. = "We should leave him alone."
- ¿Quieres entrar? = "Do you want to come in?"
- Trataré de hablar con ella. = "I will try to talk to her."
Now your example says:
Creo que estoy tratando de forzarlo.
This is actually fairly complicated because there are three verbs and each one is subordinated to the previous one.
- Creer is followed by que, indicating that what follows is a subordinate clause (as in English when preceded by an optional that).
- The subordinate clause, estoy tratando de forzarlo, has a main verb tratar in the "present continuous" pseudo-tense. Tratar is commonly followed by another verb in the infinitive, subordinated using the preposition de, just as in English you would use to in try to….
- Finally there's the verb forzar with a direct object pronoun attached to it.
In each step you must know how the verb works with other verbs. Creer is often followed by full subordinate phrases, and tratar de is followed by infinitives, but the reverse can also be true:
- Creo estar tratando de forzarlo.
- Creo que estoy tratando de que se afloje. (aflojar = "to loosen")